Rotting Leaves

I would remove them.

Len

I have a 20 that i've not pruned in nearly a year. It's got a bed of swords under a mat of hornwort (okay, I remove some hornwort once and a while). The sword leaves die and break down into fiber skeletons but the tank keeps going fine and has less problems than my big tank that gets more attention/light/ferts/ect. There is some hair algae, but killies like it for egg laying so I don't mind.

This tank is solid plants, so you might not get away with it in a less dense setting.

Leaves from hygro ssp will grow into new plants. I used to leave them, but most hygros are prolific enough just with runners.

You can leave the leaves in the tank if you want to... but the decaying leaves will add to the organic build up in your substrate. Over time, this can lead to old tank syndrom in a planted tank. Quite a few plant keepers break down part of their substrates every now and then to remove excess organics from them. So it only makes sense that by removing loose leaves and such early on will help slow down this process. Removing dead/dying leaves will also remove any potential food sources for snails.. which should control their population (that is, if you have snails in your tank to begin with).

HTH
-Richer