When I was at the elementary school, a comma before ma was considered compulsory. On the other hand, a comma before e was a terrible sin worthy of a red sign.
If ma connects two adjectives
Kyriakos è brutto ma simpatico
(I needed a name, every connection with real people is incidental
;-)) no comma is actually required. When ma connects two propositions, then it's usually preceded by a comma.
In your first example, I see a slight difference between
Essere sinceri è importante ma non troppo.
Essere sinceri è importante, ma non troppo.
Adding a comma forces a pause, so in the second case I see a grin in the face of the speaker. Just a sensation, probably.
The second example is usually written with a comma and I'd use one myself. I wouldn't goggle at a missing comma, though.