It's a subjective thing handling!
TBH You'll never notice the difference on a road car, you might feel a difference if you go for big changes, or they might be in your head. Does it matter? Just go with what you want.
FYI I on my MX5 I had ET15 wheels on the back and ET40 wheels on the front (same widths) handling felt fine. Was better than before if anything, but then it's not just the offsets that changed, I had wider wheels/tyres etc so it wasn't a fair test. Looked cool though!