It's interesting how CL's fit diferent people differently. I bought my first 2 pairs tts and they stretched so much I had to sell them to a g/f. I have bought all my recent CL's at least 1 size down (1.5 sizes down in my suede So Kates) and after a few wears they mold to my feet and I NEVER have heel gap anymore. I remember one TPFer saying she wore a size 8 and the SA at the CL shop recommended a size 6 !
I wear 100s especially the Pigalle every day for work running all over town for meetings so I do not mind a bit of heel slippage coz its still walkable elegantly.I think the biggest factor is how much time does one end up wearing the CLs. For me it's very infrequently or less, so there is not really enough time to "stretch" and every time I do wear them it's the pain all over again. Thus, for my lifestyle, I shouldn't have sized down.
However, if, as most people in here, one wears her CLs a lot, every day, to work, to school, in outings etc, then I guess it's different. I feel, though, that even in this case I might have preferred a liiiitle bit of heel gap over the numb and crunched toes.