You all might think that I may be doing it wrong, but I tend to leave it up to my opponent to dictate the number of spells I switch out. If the battle calls for it, I don't see any harm in dropping a bunch of High Magic stuff.
Typically, I drop 3 spells per battle. The psychological effect of this on the other player is often priceless, even for having only generated mere signature spells. It can turn your opponent's otherwise perfect positioning & coordination to a devastating trap.
As a rule, I always pick up Spirit Leech. I haven't gone a single battle without doing that, and it's never done me wrong. At its worst, it draws a lot of dispel die. At its best, it lengthens your magic phase & takes monsters/characters off the board.
But, I've also had battles where I've dropped upwards of 5 spells. I've had battles where I've generated almost every spell from a single lore. I remember adopting more & more of the Lore of Fire against High Elves as one game progressed, something that a lot of people seem to consider heresy.
Man, the look on my opponent's face when I started my turn 5 magic phase by casting turbo'd Fulminating Flame Cage with three dice on his very static & shooty Sea Guard. He said, "Uhh.. Yeah, they're not moving anyway. I'll save my dice." I then proceeded to draw out his dispel die as usual, 'til I had 2 to his 0. For the first time in the entire game, I casted Wind Blast on them with my Lv 2 Priest. I thought he'd flip the table when most of 'em dropped dead from that combo. D6 S4 with the initial cast, D6 S3 with Wind Blast, unit-wide S4 from Flame Cage exploding due to movement. All of that for one dice less than Fiery Convocation. The first time they moved in the game, they were running off the table. Ahh, fond memories. 8)
Ack! Got sidetracked. Sorry. Anyway, I think it boils down to the situation & how your opponent is playing every single time. No right or wrong way of doing it, only right or wrong spells, and right or wrong timing. *shrug*