It's still recommended to use matching pairs for RAM. Pairs of sticks allows the CPU to access it faster since it can access both sticks at once. It won't consider them a "pair" unless they actually are: i.e., same capacity, speed, etc. Naturally, the best way to ensure this is by buying them as a matched set rather than trying to mix and match from a grab bag.
Also, your board will have two sets of slots (assuming you have 4 RAM slots) and it matters which ones you use if you're not filling all four at once. Check your board or look online but you'll have an A set and a B set. In the last system I built, the A set was actually the second and fourth slot going right-to-left as opposed to just starting from the first slot.