Hitch Rack Tips
Trailer hitches are made specifically for each make and model of vehicle, and are bolted to the chassis of the vehicle. Sometimes, drilling holes in the chassis is necessary, depending on the vehicle. Also, some cutting of the lower bumper fascia is necessary, but this is rare.
Trailer hitches come with class ratings, which relate to the weight rating, or number of pounds the hitch can tow. Class I is the lightest rating, Class II can tow more, etc. On top of the class rating, hitches come with individual ratings. Mine, for example, is a Class I rated to 1,500 pounds GTW (gross towing weight).
The number of pounds the vehicle itself can actually tow depends on how much power the engine has, and what the vehicle manufacturer rates that vehicle for. For this reason, hitch manufacturers only make a class of hitch that matches what that make/model of vehicle can actually tow. So, even if I wanted to put a Class III hitch on my Civic, none of the hitch manufacturers are going to make a Class III hitch because even the most powerful Civic Honda makes can still only tow up to a Class I. So you can only get a Class I hitch for the Honda Civic. (That said, if you really really want a Class III hitch for a Honda Civic, you may be able to get one custom made if you can find a hitch shop willing to do it. I've only ever heard of one [Sealand in Richmond, BC], but there are likey others out there that can do it.)
The reason the class of hitch is important is that it dictates how many bikes you can carry. Class I hitches can only carry two or three bikes (depending on the individual rating of the hitch), Class II hitches can carry three to four bikes, and Class III hitches can typically carry four to five bikes. So, if like me you have a Honda Civic, which can only tow up to a Class I rating and thus only has Class I hitches available for it, you can only carry up to three bikes on a trailer hitch mounted rack. If you want to carry more, you will need to invest in a roof rack.
An important thing about mounting a trailer hitch to your vehicle for the sole purpose of carrying bikes is the vehicle manufacturer's warranty, and the re-sale value of the vehicle.
Honda, for example, does not warranty the Civic for any towing whatsoever. Thus, if I had transmission problems (as an example), and the dealer thought I was using my Civic for towing, they could deny me warranty coverage. However, in order to tow a trailer, you need the proper wiring for brake lights and turn signals. If my vehicle is not properly wired (it's easy to check if you know what to look for), how could I ever have (legally and safely) towed a trailer? For this reason, I do not have the wiring installed, nor do I need it just to carry bikes.
That said, if a drivetrain problem ever does develop in my Civic, just to be safe I would likely remove the trailer hitch altogether before taking the car in to Honda.
Regarding re-sale value of the vehicle, any vehicle used for towing will generally be considered to be "worked harder" than a vehicle not used as such, and thus would be worth a bit less. Again, if the wiring is not installed, it could not have been used for towing. Hopefully the prospective buyer trusts your word, assuming you are being honest. Again, removal of the hitch before selling might be a good idea just to avoid questions.
This is all assuming you only want the trailer hitch for carrying bikes and not for actual towing. If you are towing, you will need the proper wiring, and all of the aforementioned points are moot.
Well, actually, that is not exactly true. Technically, it is illegal to obstruct your tail lights and license plate like you do when you carry bikes on the back of your vehicle, either with a hitch rack or a strap-on trunk rack. In order to properly comply with the law, you would need to install a set of brake lights/turn signals and a license plate relocator behind your bikes in plain view (there is at least one company producing such a device). In order to do this, you would need the proper trailering wiring. Back to square one.
I have never seen anyone use a brake light/turn signal relocating device while carrying bikes on the back, nor have I ever heard of someone getting a ticket for not using such a device. I have, however, heard of people getting tickets for obstructing their license plates. Specifically in Vancouver, but also in other areas, there was a bit of a blitz of media attention and police crackdowns on people obstructing their license plates with bike racks and bikes a few years back. The rack manufacturers responded with license plate relocating devices or mounts on their rear mounted racks, although most are still sold without them and/or most people do not use them. Basically it has boiled down to this: generally, you will not be pulled over or ticketed for blocking your plate or lights with a bike rack and bikes, but if you do anything to attract attention (poor or aggressive driving, for example), it can be used as an excuse to pull you over and/or give you a ticket.
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Up until this year I used a hitch mounted rack on my Civic (pic left). This is the kind of bike rack setup you want if you are on a budget, but you value both your bike and the paint and bodywork on your car. It is a 
For the serious setup, get a