My RE only does 5DT now b/c they have a higher success rate than 3DT and he believes that if the embies don't make it to Day 5 they wouldn't have survived anyway.
For whatever reason, I am a low responder as well.
My first IVF: 6 eggs (all mature), 6 fertilized, 5 made it to day 3. At this point RE was still doing 3DT, so we transferred 2 embies, resulting in my 22 mo old son! We waited to Day 6 w/ the remaining embies and froze them. (Did FET in May/June w/ these that didn't work.)
2nd IVF (current cycle): 9 eggs, 8 mature, 5 fertilized, 4 made it to Day 5. 2 transferred on Day 5. 2 left until Day 7 to decide about freezing, but they didn't make it.
It is a risk w/ fewer eggs to do a 5 DT.
I think you need to take into consideration the quality of the embryos, the numbers, and your emotional state. What if you get to Day 5 and there is nothing? Do you believe that the cycle wouldn't have worked b/c the embies weren't good? Will you question whether they would have done better inside of you? In the minimal, non-scientific research I have done, Day 3 transfer have about 40% success for my age group (under 35) and Day 5 have 70% success. I was willing to take the risk b/c my dr was a firm believer.
As with all this stuff, sometimes it just feels like a crap-shot.
Me: 33 DH: 37, Low Morphology, DOR
IVF#1 2/08: BFP! 11/08 Healthy Baby
FET #1 5/10: BFN
IVF#2 8/10: BFN, 2 Day 5 blasts
IVF#3 11/10: BFN, 3 Day 5 blasts
IVF#4 2/11:BFN, 2 Day 5 blasts
IVF#5 4-5/11: BFP!!! 2 3-days (7 cell, 8 cell) 5/9=866, 5/11=1595