I'm guessing that you have a G5 since you have 8 slots and are talking about 1gb sticks, although that information is generally useful in answering this question.
With the G5, the slots are "paired" and correspondingly all the sticks must be also. The two center slots are paired, as are the slots building out from this. If memory is not paired correctly in a G5, the sticks simply won't be recognized. If all are recognized, you don't have anything to worry about.
There were some nuances in pairing ram in older Macs, but in general the consequences are either that a particular bank won't interleave(reducing performance) or the bank will be recognized at the capacity of the smallest stick. Again, these don't apply to a G5.