Things will be off the grid, even below a grid unit of 1. Also you should start with grid units of about 16 or 32 so brushes are easier to work with while your map is in alpha stages.
Hard disagree on this one. Save very often; Ctrl-S is your best friend in any crash-prone program like Hammer. That way, if Hammer crashes, you'll lose very little work and can easily pick up close to where you left off. If you go for long periods without saving, you're just massively increasing the odds of losing hours of work due to a random crash.
Personally, I always found it demotivating having to relaunch Hammer every time it crashed, to the point where 'little work' is what actually is done in a specific timespan. However I didn't necessarily mean doing all of what's planned in the same session, but the majority of it or so.
