This isn't 1993 memory is cheap.
Right now I think it makes sense to just fill all slots on the board and you don't have to worry about it for the lifetime of that system.
Memory prices also tend to go up and down within a year which is another reason to buy when its low.
You don't want to be stuck in between a transitional phase for memory either like going from DDR2 to DDR3. There are quite a few people looking for DDR2 memory now which is very expensive.