No, no , no, no.
Unlikely you will be allowed in the main dining room on a formal night in smart jeans and a tee shirt. I also have a tux and take it when cruisng but more and more are in smart suits and dinner jackets, but don't let that put you off taking it.
I thought I did reply to this but most ships will allow this on a casual night but not formal or informal evenings. The white tux is not as prominent as it was, more popular in the caribbean. I do take mine with me whether Med, Caribbean or Ausralia.
Smart jeans and t shirt ok in all but the waiter service restaurant. You can do so but may feel a little out of place as we noticed everyone dressed smart casual.
"Smart jeans" is a contradiction in terms....Nei.