Not every one, but the key ones.
-Skip the area 9, 8, 7, 6, and 3 (purple and sea) gates completely. Well, unless you're going to go to area 9 from one of those places, but really for those I think another path would be better. I could check up on this, I suppose.
-You have to get the one on the way to the area 2 boss; it's the only way to unlock the rest of the main road.
-The gate room in area 2 (IE the switch just after the Polar bear miniboss) can be ignored if you're not going to area 9 from area 2. This is why some planning for that is needed.
-The gate room in area 3 that leads to area 8 is good to get because it's not far from the boss of area 3 and it's also really close to area 8.
-The gate in area 4 is sort of iffy, I guess. There's a switch on the rainbow road just around the area 4 entrances. The issue there would be if it's faster to a) go straight to the boss from that mirror room or b) get to the area 4 mirror room and activate that (then activating the area 1 mirror room it leads to) and back. I guess I'm not that sure about this gate.
-On that note, I believe you want to get all the big switches in area 1 simply because each one leads to multiple areas.
-The area 5 mirror room should be activated because the entrance to both the area 5 castle and the area 7 castle are right next to it.
I suppose I could do some testing to see if I can find the optimal paths. I guess I'll start looking at that now.
EDIT: Just did some testing. I'd post movies as well but they always desynced on me. Anyways, on getting to area 9:
From area 2, it's a rather long path to area 9. The mirror to the area 9 mirror room (which is the choke point of the whole level...you have to pass through it to get to the boss) is 3 mirrors away, but you must go through a complicated and slow water room.
From area 4, it's a medium path, but you must sacrifice whatever ability you have to get the small ability, after which you have to fight a Dog Fighter boss (with nothing, sword, or slice). In area 9, you begin 2 mirrors away from the area 9 mirror room.
From area 7, it's a short path, you have to fight a Polar bear miniboss, and the mirror room is roughly 2 mirrors away (but you'll have to leave & re-enter a room from a certain mirror).
Given that, I'm thinking the area 7 path to area 9 is the quickest. This means you'll want to activate both area 7 mirror rooms. This is actually pretty easy because...
-When you first enter area 7 (or rather, just before you do), you should be just outside the ice castle. You run past it, enter the mirror just beyond it (you're in an ice cave). Run to the first mirror, enter, you're at the "far" area 7 mirror room (which leads straight to the nearer area 7 mirror room, which you activate). From there, you can either continue through the cave to the area 7 boss or go in the left door of the "nearer" room to get to area 9.
