Most Helpful Guy
If you've ever heard the term "snowballing", this would be equivalent to deliberately pushing the snowball faster down the hill. Asking isn't actually going to make anything better, especially with a person who hasn't given you reason to doubt them. It's just going to ease you for a moment, before you need to be reassured again.
Most Helpful Girl
Do you legitimately suspect he's cheating on you? Do you have any kind of proof? If you're going on just the dream, than I don't think you should ask. If he isn't and you accuse him it could cause problems.