does leaving gps on all the time kill the battery
Not in my experience. Actively using the receiver (i.e. driving with Google Nav for extended periods of time without a car charger) will kill the battery though.
Well, generally, no, it should not kill the battery, however, if you have apps that periodically poll the GPS for information, like the Google Browser when it hits Google, or Locale, or such, then it eventually does drain the battery over time because you are actively hitting it for information. As Takeshi has pointed out too, Navigator will heavily use it because it wants to constantly update.
There are 4 radios in the device, the phone transceiver, the wifi transceiver, the bluetooth transceiver, and the GPS receiver. If battery conservation is anywhere near to top of your list of priorities, whatever you don't need should be off. Which of them you decide to power up should be predicated on what you intend to do with the device.
I have GPS, Wi-Fi, and BlueTooth on all the time. I use the phone and mess around with it all day. I charge it late at night with no problems of it not making it through the day. I'm not browsing and stuff like that hours at a time. I do text and some browsing from time to time, get my e-mails. A lot are from this forum even though I'm on my desktop I still get it on the phone so I check to see what's going on here when I'm reading another thread.
Yeah I leave mine on all the time. I also use locale which turns on GPS for literally about 5 seconds only, every 10 minutes to check my location and determine if it has to change my phones settings. I do leave wifi and bluetooth off. There is probably a little battery effect but I have no problem making it through the day with the batter unless I use it a lot then I charge it in the car on the way to work