I normally stick to zero as a garrison. If I have 30 planets each producing a Constructor, let's say 2-3 Constructors per turn across my empire, then I can at will switch that production to military vessels and increase military spend at the same time. As long as I have some cash reserves I can purchase ships at key spots.
This way, when I am DOW'ed by somebody then by next turn I can have say 6 ships in the 120-150 cost range, more if I switch to cheaper ships (fighters, corvettes) and/or buys ships outright. So far I've not come across an early/mid-game situation where the AI attacked my home systems faster than I can build a defence that can hold.
The money you save in upkeep makes a real difference IMHO.