Don't get hung-up on detail, but don't use that as an excuse for sloppy, or lazy mapping.
Solution? Use 16 as your smallest grid size when first making a map. You don't need any more accuracy for detailing, you've just started! The *only* exception is when placing game-critical props (doors, cp models, payloads, etc), and then use Ctrl - B to make it snap to the current grid.
Also, always, *always* line up your walls, floors, levels, etc. in the local area. Not doing so from the second you start mapping can cause vicious /RAGE* later on when you're tweaking/styling your map and bump into dozens of void leaks, dozens of world/detail/brush lines making quick editing difficult, etc. Personal experience over 3 years with Hammer.
*see Left4Dead