To make your decision, you need to first understand why you'd want to be grabbing in the match-up. A few points you need to consider:
-Peach is in her shield for a considerable amount of time in this match-up. She can't risk getting hit by dairs, or miss CCs on nairs. Both aerials lead to devastating follow-ups on hit if the Peach doesn't react in some kind of way. Shielding is a good option for Peach because her nair is quite fast and works defensively oos.
-Uthrow Uair does not combo at any percent on Peach. However, I'm sure you know that because of the horrendous speed on her double jump, she is almost helpless once thrown in the air. Her only move that hits below her (besides strangely angled bairs) is dair. But we all know what will happen if Peach decides to take that path and combat your uair with her dair. Bottom line is, even though uair isn't 100% guaranteed ever - it still works alot of the time because she can't really avoid it.
-Peach is floaty and dies early on many of the low ceiling levels.
-Peach has the option of relying on crouch cancels to secure some good damage against Fox. Watching for blatant crouches and other CC baits can lead to free grabs.
etc.
The overall utility of the grab in this MU is too great to be ignored in IMO. If not for uairs, than for fthrows to pressure the Peach to the edge. The simple fact Peaches like to shield should be enough for most Fox players to want to throw it out. Also, it should be noted that if you know which direction Peach will double jump, you can fullhop early bair and beat some of her aerials as you are rising.
Grab = Damage. Try to judge when best to use it, and take true advantage of your position below her.