I've consistently modified my dwarves to give them human lifespans (maxage 60:120) so there's always some healthy "churn" in the population to keep it reasonable. It's also satisfying to read the slabs that say the dwarves "died peacefully".
Why on earth would you be satisfied with dying 'peacefully'? Bah. Nearly all my dwarves die to magma related accidents or megabeasts. Then it's satisfying to read the combat reports. :p
I find around 150 dwarves is good. I play with no traps or marksdwarves, so I generally have a large military. Once I had a population of 300, and just under half of them were soldiers. Eventually the micromanagement just gets ridiculous though.
I could imagine you would have a few problems with tantrums if you have a sudden wave of death.
Or undeath if a necromancer sneaks in.