At 50 feet or less, it's not really a critical factor. Sure, every bit helps, but unless you are listening to pretty weak or distant signals to begin with, the loss usually isn't enough to notice, even with really cheap coax. However, if you are indeed trying to receive really weak or distant signals, especially anything in the 800mhz range, or if you are going over 50 feet in length, then it pays to get quality.
The chart that scannerpro gave you is helpful in putting some quantitative info in your choice. Unfortunately, that chart doesn't cover the LMR series of coax, which is arguably some of the best choices, and more widely available than some of the obscure RGs. Here is another chart that covers the LMRs:
http://www.hamuniverse.com/coaxdata.html
LMR-400 is widely considered to be the best compromise for scannists. It is much lower loss than the RG series coaxes, but not so thick and/or stiff that it is too difficult to work with. And it's pretty commonly available, even at ScannerMaster.com. You can get better coax, but at fifty feet, there's really no reason to. However, if you are going to go overboard on anything in your system, then coax is indeed a good thing to do that on, lol.