I usually start off sleeping on my front but by morning (if ds hasn't woken for food in the night!) my breasts are usually too full for it to be comfortable. I've been sleeping on my front since ds was born (he's now 16wks) as I'd got SO fed up with having to sleep on my side while pg. I haven't had any problems with blockages or anything but then maybe I've been lucky !
Never imagined how much I would miss sleeping on my front until I was pregnant and definitely think it contributed to blocked ducts in the early days of bf. Have only really been comfortable in the last few months and DS is now 14 months old, am still bf but it doesn't seem to cause problems now (hope they aren't the proverbial famous last words)
I did as soon as it was comfortable enough - around three months I think - when I got to the point where I didn't really get engorged anymore. Brief phase around 6 months where DS had a big growth spurt at the start of weaning where it hurt too much for a couple of weeks but all fine since then. Thank goodness, I can't sleep properly any other way!