I assumed that you meant DIMMS, rather than SIMMS - in which case you would need matched pairs.
I think most people are quite loose with the definition of "matched".
Initially, I meant that it would be a good idea, if you were buying DIMMS, to purchase ones that were from the same manufacturer and ran at the same speed.
Here, in the case of SIMMS, you would ideally need identical pairs - size, speed and (preferably) manufacturer.
If you do need SIMMS, make sure that they match the existing ones (EDO, Parity, etc), or you may see unusual error messages.
I hope this information helps