There is a tradeoff. You need to decide if it's worth it. What you might be giving up (depending on your existing configuration) is dual or triple channel mode for your RAM. Effectively, it will make all of your RAM operate more slowly. In most cases, having more RAM will outweigh having faster RAM, but there comes of a point of diminishing returns where the speed of the RAM starts to play a larger factor.
It's hard to give actual numbers on where the trade-off point is because there are so many factors: the base (single channel) speed of your RAM relative to the speed of your disk subsystem, the amount of each and how much RAM you are adding, how much RAM your application actually wants to use, how the cache hit/miss rate changes as your approach that last value, how good your memory controller is, etc.
All else being equal, faster RAM is of course better than slower RAM, and you should strive for matched sticks where possible. But in general, your first job is to get enough RAM in there that, once loaded, your application only rarely needs to hit the disk for page fault reasons. Worry about dual or triple channel options after that.