I try to buy them anytime I see them on sale - it's easier (psychologically) for me to change the blade if I have a bunch of spares on hand rather than just one or two.
I used to wear things like my rotary blades down well past normal use, but I've started to ask myself why? I just add fatigue to my poor wrists and elbows that way. Blades are easier to replace than elbows!
So now I "treat" myself to things like fresh rotary blades and needles anytime I suspect they're dulling down. They don't cost that much and there are better (wiser) places in my budget where I can cut corners if I think it's necessary. (Like not buying so much fabric? Naaaaaahhhh, that's crazy talk!)