To the best of my knowledge (and there's a lot of misleading information out there) a Li-ion battery ages with age, not charge/discharge cycles. Nickel-based batteries age with number of cycles.
If the battery gets hot because it's always being charged, that might shorten it's life. Li-ion chargers are very sophisticated though and probably don't generate much heat in the battery when it's being kept topped-up.
I'd be tempted to flatten it and recharge it every now and then, but more because I feel it's the right thing to do than because I have any hard evidence that it will help.
It's much worse to leave it flat than to keep it on charge though, so you're erring on the right side of caution.
This website is the best I've found about batteries :
Is lithium-ion the ideal battery?
cheers,
Andy.