How should different sized RAM be added to iMac?
I'm going to take WAGuess* and suggest that any gains (realized or not realized) due to memory-size matching will pale to insignificance beside the gains due to tripling your memory.  My rule of thumb for processor speed is that our perceptions tend to be more logarithmic than linear and that we don't really notice a speed increase until it is 50% - 100% greater than what we had before.

If you're in a situation where a few - consistently acheived - percentage points matter, such as frequently or perpetually running long, compute-bound jobs, and and where finishing a job a few minutes earlier might have a significant benefit for someone, then there may be an advantage to optimizing your memory placements.  Otherwise enjoy the speed of you newly augmented machine!  You've already got 99++% of the benefits to be had.

'*' That guess isn't completely WA'd: I've already looked into using mis-matched memory sizes on two of my older Macs, and the above is what I concluded from at the time.