Walls/floors will weaken the signal some. Whether or not it weakens it enough to affect your wifi connections really depend on the cameras and wifi router, as well as how much distance and material is between them. Some "wireless" cameras (especially the cheap ones) have notoriously poor wifi reception...so bad that people have to mount them within a few feet of the wifi router in order to get mediocre results. Some routers are also better than others as far as signal strength and bandwidth go. I have hardwired all of my IP cameras because I am using them for security...and I'd rather always have good framerate and a solid connection. I would also imagine that using the ethernet connection would reduce the amount of heat generated within the camera...and might help with stability. In SKJM's post concerning recommended cameras (
http://skjm.com/forum/index.php?topic=908.0), they mention that they have to reboot some of their cameras every few weeks. I don't know for certain if it makes a difference (wired vs wifi connection), but I can tell you that I never have to restart my D-Link or LOFTEK (FOSCAM knockoff) cameras...and a couple of those cameras are in a house that is being remodeled (temps vary from 20-110 degrees Fahrenheit, as I do not run the A/C or heat). These cameras have been working for over a year in those conditions (yes, I'm slow at remodeling). So it will be virtually impossible for anyone to say "yes, that will definitely work." If all devices used lived up to their wifi claims, you shouldn't have any issues (assuming you don't live in a huge house). But you'll have to test it and see what works...and be willing to send back wifi cameras that under-perform or consider running ethernet cable for them.
