Yes, once the film is in the tank, with the lid secured, the lights can go on.Btw, if I put enough towels underneath my bathroom door to block out the light, I don't need a changing bag and can turn on the lights once the film is in the developing tank... is that right? sorry! I know it's a newb question
You can block the light under the door with towels or whatever might be handy. The darker the better. Also pay attention to the other three edges. If you're looking to use the bathroom for loading/developing regularly, putting weatherstripping around the door, and a door sweep on the bottom should take care of the light, without the hassle of towels or permanent changes. One way to test is spend 5 - 10 minutes in the room with the lights off, and see if you can then detect any light coming in. If you can't, you're set. You can get away with small leaks, but there isn't any easy way to describe what might or might not be a problem.
Another test, after 5 or 10 minutes, you should not be able to see your hand in front of your face, for example, if you can, something is leaking too much light.
If possible it helps to have any lights off in the space outside the door. Some darkrooms can only be used at night.