Check out www.lycheesonline.com for lychee care. He has an entire chapter on soil and mulching. Mulching is always a benefit.

I'd imagine that mulching around every plant/tree/shrub is very useful/important.

There are a lot of theories that say citrus shouldn't be mulched but some people mulch them and they do real well.

I mulch everything except for my citrus. I have both my lychees and my sugar apple mulched and they are doing well. I make sure that I keep anything from growing around the citrus though. When mulching make sure to keep the mulch a few inches away from the trunks of your trees though because it could cause problems.