Lowering your intensity on any pool would drop your rejects.
I believe this to be specious advice, though I cannot with absolute certainty claim otherwise. Read this, investigate further, and then decide for yourself.
Intensity is used to spawn GPU threads as a simple 2^value setting.
I:13 = 8192 threads
I:15 = 32768 threads
I:17 = 131072 threads
I:18 = 262144 threads
I:19 = 524288 threads
I:20 = 1048576 threads
Notice how the higher settings increase thread count tremendously.
In short, if raising intensity increases your rejects my x%, but it simultaneously increases your hashpower by more than x%, then you're better off despite the rejects. And there are other causes for rejects too, outside of your control. As such, you have to expand your time horizon before jumping to any conclusions.
It seems odd on the face of it, but decreasing intensity can actually result in higher accepted shares as well as lower rejects despite what may or may not be a lower raw hash rate. The formula for time taken to complete a specific set of work sent to the GPUs is expressed as ((2^I) / (hash rate *1000)).
Assume that a card is giving you 600KH/s.
At I18 it will take 0.4369s
At I19 it will take 0.8738s
At I20 it will take 1.7476s
"Aha" you say, "but it will take a little more than half the time to complete half the work each time you drop the intensity because the hash rate will drop slightly". True because there is a need to send work back and forth more often, and that introduces some slight inefficiency. But you also have a good chance to produce an accepted share within that smaller block of work and submit it before the work is stale, and that's why you can end up with both higher accepted shares and lower rejects.
For reference, I ran 9 identically configured 4MH/s rigs for 24 hours, each pointed at the same pool that gave them the same work difficulty (1024). The results expressed as average per rig were as follows:
I18 Accepted Work: 3,371
I19 Accepted Work: 3,405
I20 Accepted Work: 3,391
I18 Reject Rate: 1.84%
I19 Reject Rate: 3.17%
I20 Reject Rate: 4.34%
Since we get paid for accepted shares, I19 was the clear winner despite a marginally slower hash rate (approx. 4KH/s per GPU).
