I do exactly that - recently splurged one hoard for my second-last X-23 cover, but I wouldn’t have broken discipline for Howard the 
!
Actually, if you need the ISO (and who doesn’t!?) then maybe having that ISO now instead of in a few months’ time would allow you to better place in PvP/PvE in the interim period, leading to a net difference in total rewards earned in-game between those who spend their tokens as they go vs those who save.
Personally, I save my tacos because I still need plenty of 4* covers and, rarely, will splurge on tacos to pick one up that I’d really like. More important to me than ISO, for now (barely).