Thanks everyone for your help!
Sorry I'm not making this clear. What I meant was that since I was trying to build a Instrumentation Amplifier, which involves 3 OpAmps (using 3 out of 4 from LM324) connected via resistors, like this one from wikipedia:
From what I know, if I want to reduce the common mode gain and offset errors, the resistors R1's, R2's and R3's should be exactly the same. I don't have a electronics shop, I only have a couple resistors of each value, so trying them one by one is not really an ideal solution. I guess I can use pots, but that'll add 3 pots to the design, plus the one already on the Wheatstone bridge! Buying some 1% resistors may be the best solution.
I get the feeling that I'm following the book too much, trying to build a perfect amplifier. A little less CMRR may not matter in the end at all, since it's not like I'm building a precision instrument.
-Z-