Source as in whatever the source is... for me it varies... MPC, mixer or tape machine... nowadays it's usually the tape machine.
With the tape machine i never use the gain on the apogee, i even have to turn down the outputs of the tape machine, depending on how hot i record onto the tape.
Generally i try to have the source as loud as possible, much like an MPC that would play at full velocity. I think it really doesn't matter too much with the apogee tho, since the preamps won't really color the sound if you turn up the gain. Think of it as a helper in case your source isn't loud enough.
I said try it yourself so you could compare the results. I believe it makes no difference (or almost none) if you turn up the gain (and have your MPC turned down) or not (and have your MPC turned up more)... but why don't you try it for yourself?
If you are looking for a "golden rule", i guess there's none. From what i've learned, when recording a signal, you should always have a good strong signal and turn up gain only if needed (unless we are talking about preamps that color the sound and you want that coloration)... but opinions may differ.

"It is no measure of health to be well adjusted to a profoundly sick society." - R. D. Laing