It's a legit question. Timbre matching is not an issue with a stereo (2 pcs) pair of speakers. Usually manufacturers will sell a pair of speaker with matched enclosures and drivers, crossover, internal wiring etc. Thus, it is likely to be timbre matched already. What you must be referring to is a multi channel setup.
Timbre matching is a process of having similar sonic characteristics between the fronts, rears and center(s). There are cases when even speakers of the same brand are not timbre matched for a 5.1 channel setup. The simplistic explanation is if the front speakers are inherently "dark" or "bright" sounding, try to at least approximate that character with the center. Personally, I don't think its as critical with the surround speakers though.
Even as my 5.1 channel speaker system was sold as a matched system, my center still has timbre adjustments in case I decide to change my fronts. As you turn the knob, the sound coming out goes from a rather dark muffled quality at one extreme to a hyper detailed shouty bright nature at the other extreme.
I hope that answers your query...