Don't worry - it'll be fine
I have a 15" TiBook which is almost always plugged in and sitting on my iCurve. I have it set to sleep after 30 minutes and the monitor goes to sleep after 20 minutes. Also, I normally close my laptop after use making it go to sleep. It is perfectly fine to do this and it will cause no problems. When the computer goes to sleep there is no real activity so the fans turn off and the processor doesn't work. The battery automatically allows itself to fall below 95% before recharging so that you don't have any battery problems. Don't worry about leaving it asleep all the time.