Let me put it this way...Do you also make players pay to tailer their clothes when a physical stat changes? No? Odd...Why not? It would make sense that going from Hulk stats to Bruce stats from a spell would cause your pants to be quite a bit looser. Would also make sense that Johnny upping his natural Strength stat a point or two would need to get refitted for all his armor and clothing. Right, because it's needless bookkeeping in an already rule-bogged system.
This is such a pointless argument. You do you omae. We'll be over here not being phonebooked by our players.
Depends on the type of outfit. If it is something that is custom fit, like a Second Skin, or a professionally tailored suit? Absolutely they'd need to get it tailored to fit if something big changes. As for your example, the spells that augment attributes don't cause physical changes, but rather use mana to reinforce things. You're thinking of physical manipulation spells rather than health spells. So Shapechange? Yes, you'll need new clothes. Flesh to Goo? You are going to have to dress up in icky clothes when that's done. Petrify? No prob, though the clothes aren't going anywhere unless someone cuts them off or you turn back to flesh and can move again.
Now, if Johnny is hitting the gym and getting buff, then yeah, he'd need to get his tux tailored to fit, but off the rack jeans and an armored jacket? No. Why? Because off the rack has a large range of body types in each size that will fit it, while custom fit and professionally tailored stuff often needs to be adjusted for minor changes or it looks awful. Oh sure, you could probably wear the suit coat that fit perfectly before you got some more muscle, but it would look wrong (social modifiers, and making you look like you don't belong), and it might rip up when you start doing anything strenuous, like dancing or combat.