i raise 99+ EP every time. limper or not.

1. value; knock out small overcards.
2. you def dont want to limp in with multiple LAGs behind you. you'll get raised and re-raised. then you'd probably have to fold in most cases. which sucks. i can't think of a reason to limp in.
3. bet until told otherwise, with passive players. proceed with caution with aggressive players. it depends.