As a general rule, I assume that a spent fate point / tag / whatever has to do something. If someone tries to spend a fate point but would still miss, I let them know, and give them the option of either having the point refunded, or spending more points to hit. I'll also refund a fate point if someone would have taken out the target without it.
The exception to this is if the defender also has fate points or tags to spend. If you miss by one, spend a fate point to hit, and then the defender spends a fate point to be able to dodge - both fate points are gone.
Magical maneuvers are a wonky thing, though, since it's a set power against some (un-rolled!) defensive skill. In this case, I'd typically tell a player outright "You're going to need a power 7 effect to hit this guy with your maneuver-that's-defended-by-athletics" - or, at worst, allow the PC to roll an assessment as a free action to know the right power value. In the latter case, I could see allowing someone to eat backlash to get off a spell that can't work even at its best, if they failed that assessment.