I would respectfully disagree with Gordon.Moss around the trunk is more likely to cause problems with the tree.It could hide problems with the base of your tree...it could keep the trunk base perpetually moist..and it could hide insects which might cause problems.So....I would clear the moss from around the trunk and leave it on out towards the edges of the pot.
Rockm's statements bring up a couple of things i've always wondered about.Why,if one waters thoroughly,does moss hinder the wetting of the soil?
and,...if one uses a well draining soil,why would you worry about moss keeping the soil too wet?Bonsai soil loses moisture in 3 ways that I know of....drainage,water uptake by the tree and evaporation.It seems to me that if one depends on evaporation for the drying out of soil,either one's soil isn't draining properly or the tree isn't making use of the moisture available.This could be because the tree is not properly or sufficiently rooted in or there is too much soil for the existing roots.The moss would not be the cause,but perhaps a symptom.
I live in a hot,arid part of the country and once a tree is established in the pot properly,i have no problems keeping the soil surface moist enough to support moss without overwatering the tree.Moss is not a priority with me,but I do think that it adds to the aged and established look of a tree,so I do make an effort to establish some.
just some thoughts
andy