hey, I only used them when I got a +OPK, immediately after sex. We followed SMEP so had sex that night and the next few nights, (and one afternoon! lol) then missed a night then did it again. I used softcups for all of those times. I even went off to work wearing one once!
good luck. (and if you have EWCM for sure, and not a +OPK yet but you think its coming, then there is no harm in using one. You should be having sex every 2nd day from CD8 until you get a +OPK anyway to boost your chances and make sure you dont miss the egg!)
xx