I think ANYTHING tucked into jeans looks lame. It reminds of of Bob Sagat on Full House. It's all about balance though. If you always wear shirts tucked in, go shorter on the jacket. If you always go untucked, add an inch or two.
I think that's the problem right there!
It's not so much 'should I tuck a shirt or not with a leather jacket', but more 'tuck or not to tuck with jeans'. I never tuck with jeans, unless I am wearing wearing a jacket. But tucking with jeans can be dangerously uncool territory.