I have mine wired directly to the battery. In over a year and a half I've yet to kill the battery. I do take it off the bike when It's stored, but it stays on the bike while I'm out. I left it on the bike and powered on for a day and a half once and I had no problems with the battery.
I chose not to have it power down with a switch for a reason. Every time you stop and turn the key off the GPS turns off. If your like me that is really often on some rides, and can be a real PIA if your relying on it for directions. Personel preference I guess.