I doubt battery life would be hindered too much, but performance might be. It depends on what you have. Widgets like weather, RSS feeds, etc. can affect the smoothness of scrolling through screens.
Personally, I have something on all 5 screens. Home screen is the BLUR status, weather, messaging, and news widgets, and shortcuts to the Browser and My Account apps. Left most screen is filled to capacity with shortcuts to games (what can I say? I love my games!), left of the home screen is one big widget for MixZing, my preferred music player. Right most screen is just the Kill All widget for Taskiller, and right of the home screen is Google search, Astrid to-do list, and Calendar.