Is there any sort of rule regarding how many millimeters of space to leave when cutting the steerer tube? I was thinking of leaving 24 mm (three 6mm spacers & two 3mm spacers). That way I have room to move the stem up or down quite a bit & play around. Any insight to this would be great.
-john

You're in the ballpark. The spacers you're discussing are a bit odd in size (most bikes come w/ 1cm or 5mm spacers) but if those are what you have already then it sounds fine. If you plan on reselling the bike in the future maybe go with 3-and-change cm of spacers vs. 2-and change. Just put the remaining spacer above the stem if needed.

You're not in this situation but 4cm+ of spacers becomes unsafe for non-steel steerer tubes. Bad things can also happen if you're running a lightweight carbon steerer tube w/ no spacers, but again, not your situation.

thatll work fine as long as its a good fit.

yeah, generally speaking you should stay under 40mm and above 10mm, IMO. All depends on your fit, though. I am running 30mm