Crossing zero is generally bad play as is teasing unders. Here is why:
Crossing zero:
Packers are -3...your teasing to +3. Waste of a tease. You are buying insurance in case the Lions win on a FG you still cancel that bet out of the teaser. Regular line covers all numbers. Rarely do football games have 1 or 2 point outcomes, not enough to tease.
Much better to tease a team from +3 to +9. Obviously now you are covering your ass for a 17-10, 13-10, 16-10, 14-10 game, all important numbers.
Crossing zero is acceptable in my opinion in larger teasers of 7 and 10 points. In 7 point teasers you can teaser any dog and any favorite -1 or under to get an important +6. 10 point teasers you tease any dog or favorite -3 or less. This way you get +7.
Teasing unders is not good practice because say you have under 42. Tease it to under 48. You should just take the regular under. Your not even buying yourself a touchdown. So essentially your trying to hit a 6 point window...not good. That said, 10 point under teases are acceptable but not often.