Currently the mouse wheel in parallel projection zoom by a factor of 0.9 or 1.1 per step. This means that zooming in one step and then out one step doesn’t take you back to the previous zoom level (0.9/1.1 ≠ 1.0). The mismatch gets more noticeable the more steps in and out you take and can make it difficult to re-create previous views or create a series of matching scenes.
I suggest tweaking these zoom values to make one the inverse of the other, e.g. to 0.9 and 1.1111111. The difference to what is is now would be so small it isn’t noticeable, other than how it allows you to go back to the exact same zoom level you were at before. No slowdown or acceleration would be perceived by the user.
While at it, if the values where set to (1/2)^(1/8) and 2^(1/8) (0.9170040432046712 and 1.0905077326652577 respectively) 8 steps on the mouse wheel would exactly double or half the zoom. This is how the zoom works in various other programs.
While this would be a very subtle change it would make SketchUp more predictable and therefore easier to use. This would make zooming “just work” in cases where it currently doesn’t. In my view these more or less subtle details that makes the program “just work” without the user having to think about it is what makes SketchUp special.