I posted a long time ago regarding this issue.
If you have several different casts of plates in your gym, you should always match the plates up to be the same cast. For the very reason that you could throw out the balance on your lifts.
I see folks at the gym mixing and matching all the time and through weighing every cast of plate at my gym, I've figured that they could be off as much as 10 pounds on one side of a bench lift if they did a poor mix.
I've seen some of the individual plates off by as much as 3.5 lbs of the listed weight.
Same goes for dumbells, some are heavier or lighter than listed.
I try to reach for the same cast in the plates when I load up the bar from week to week. Sometimes I'm searching the room for the "blue plates", or the "black ones", or the thin ones, etc.
I suppose if my gym was richer they'd have all the same plates, but they don't. The weight room is kind of old equipment and an "old school" feel to it. I like that though so I'm happy to trade off searching for the plates once and a while.
It's just something to be aware of I guess...