I disagree, some boards do default to the best settings for all memory sticks in the board at the time BUT some boards do not do this, the result is either a black screen with no boot or the board will beep saying there is a memory error OR 3rd and possibly the most annoying it will run but you will get random reboots due to one of the sticks not being happy running at the assigned speed.
As some of the other posters pointed out, if you know your way around the memory settings in the bios it is possible to manually set the memory timing so that all the sticks will work reliably.
The main thing to look on the memory sticks is the CAS latency, the lower it is the higher the performance BUT mismatched with other sticks will cause lots of issues so try to make sure the CAS latency is the same on all sticks in the machine, that along with PC3 - 8500 (or whatever you have) should make it a decent match but again not always guaranteed.
I personally would check in the bios to see what the timings are defaulting to for your current memory and then buy a stick with those timings. The other way to go is to get the exact same brand and hope they came from the same manufacturing plant