Have a quick look at metalink note 175434.1, there are a number of restictions of having a locally managed system tablespace, but these all relate to having or wanting other dictionary managed tablespaces.
If you currently have a DMT system tablespace due to upgrade from a previous version that didn't support LMT there's little point changing it, but if you are starting fresh then use LMT. I think it's pretty safe bet that DMT system tablespaces will not be available in the future, so you might as well get used to LMT now.
Regarding other tablespaces, it really depends on the usage. Given the choice I'd go LMT every time!