The party line here is that speaker cable, so long as it is of adequate gauge for distance and isn't in some way physically defective, makes no difference to sound quality.
But there are quite a few people that disagree with that. In fact, the whole audiophile cable industry is based on the premise that different cables introduce different 'characteristics' to the sound. This is why $20,000 speaker cables exist. This is one of those highly debatable topics that won't ever go away.
As the other posters said, your Monster Cables are almost certainly perfectly fine for the job, unless you've got some absurdly long wire runs and very skimpy wires.
The two big problems that many of us have with Monster Cables is that 1) they aggressively try to sue other companies over perceived patent infringement, and 2) their cables are not really of any higher quality than other cables that cost a fraction of Monster stuff. My opinion is that Monster Cable stuff is a little like Bose in that you're paying a high premium for the name on the package, and not for the product contained within.
But if you're looking for good cable at low prices, check out Monoprice.com. Amazing prices and I've never been disappointed with the quality. If you want "better" cable at a slightly higher price (for whatever reason), check out the offerings from BlueJeansCable.com. Excellent stuff & great people. Or for the ultimate in matching accessories, Axiom sells really lovely speaker wire as well. Any of that would be purchased for cosmetic, aesthetic, or philosophical reasons though, and probably not for sound quality.
