Yes, I leave it on all the time since I connect to WIFI a few times/day. WIFI uses less power than 3G so you want the phone to use it.
WIFI will disconnect when the screen is off (unless you change the default setting) so it really doesn't drain the battery much. I did turn off the option to be notified of open networks as that annoys me.
I've tried leaving WIFI off and I actually use more battery power that way, believe it or not. I encourage everyone to TRY leaving it on if you have access to a WIFI network. You may be surprised at the results, and it is a lot less hassle than to have to worry about toggling it on/off all the time.
BTW, I leave GPS on all the time too.
Now then, on the Droid X because of the battery drain bug you should turn off GPS when not in use and turn off WIFI when not in use. Hopefully the update will fix this.