Depends. Are we talking a once off, or an ongoing FWB situation? I've found that casual dalliances between friends has a 'Best Before' date. Drag it on too long, and the friendship will inevitably change. An ongoing sexual relationship creates a certain familiarity that you can't really take back. That's not an inherently bad thing, but it will change the dynamic of the friendship.
That doesn't mean it has to get weird but sometimes it will. At the first hint of complication, you wanna put a fork in it, because it's done. A casual sex situation with a friend should be fun, comfortable and easy. That's the point of it. If it causes drama, pull out! (teehee) Otherwise keep the 'Best Before' principal in mind and enjoy it while the going is good.