It's varying levels of leaking from different ones. Some soak the battery apparently, lol.. some are just tiny drops you taste on your tongue, and wonder how bad that is since they tell you to wear gloves when you handle e-juice. xD Sometimes it clogs it up.
I use clear juice, it's 60pg/40vg. Earl Grey Tea... sounds weird, I know.
Always let your tank rest on a paper towel before you attach it to your batter y. Then you will see if it is leaking.
Also a tiny bit of liquid in the mouth piece could simply be a little bit of vapor condensation. Nothing to really worry about.
Twist the corner of a paper towel and wipe out your mouth piece once in awhile.