The switch to Republic is in my book a no-brainer since econ is good, and the chances of a planet defecting is minimal. I rarely loose elections twice in a row anyway, so more often than not, I'm benefiting from the econ boost provided by non-Imperial forms of gov't.
I tend to make the switch to other forms of gov't based on the following:
1) Do I need the cash, or are there other ways I can get cash? +20%, +40%, +60% econ can be very very tempting.
2) Do I have enough cultural influence? If I do, then chances are, any planets that defect will soon return to their rightful place - under my iron fist

If not, I tend to build star bases with culture modules in the sector.
3) That excess population's been sitting for quite a while in space... Maybe it's time they were given a job LOL
4) Do I have morale problems? As long as it's near the 50s I haven't had problems with Star Democracies at least. I rarely get to Star Federation until way way later at which point my econ is secure, so I don't bother making the switch.