I'll just start off by saying that I do everything ass-backwards. Tile the floor in sections, not grouting while getting wood chips and random dirt in the grout lines (reason being I'd rathe rjust grout all in one shot once all of the tile is laid).
Anyways, what's the best way to prepare the grout lines for this? Would a simple vacuuming do the trick, or do I need to get down with a wet sponge andn wipe out the lines? Either way I don't mind.
I would shop vac the grout lines with the brush attachment. If anything remains in the grout joints in the way of debris or goo, wipe it out. If you want to sponge off the tile to make sure it's clean that wouldn't hurt either. You don't want any dirt on the tile to get in the grout. It's also a good time to make sure you don't have any lumps of thinset sticking up in the grout joints that would show through the grout. Take a skinny enough old screwdriver and knock them off before you vacuum.