If you use any chem ferts you do want to flush, no matter what anyone says you need to flush. Now flushing could mean watering only for a week or so, or by running gallons upon gallons of water through pots near the end. The perfect ending to me is a big flush of about 1 gallon of WATER ONLY through each gallon of soil about 7-10 days before harvest. Then I just water normally until harvest.
You can use molasses up to a few waterings before cut, but it makes no sense to use it for flushing or for the last few waterings.
Molasses is best used as SirGanga said, begin using it when buds begin to form, about week 3-4 using it up to week 8 or 7-10 days before harvest.
DO NOT USE ANY FLUSHING AGENTS. Just water.
There will be some that say do not flush, ignore them. If you still have too much of something when you cut that will effect taste no matter what you do in dry or cure. I have to assume people like hotrod, who knows a lot, that tell people not to flush must use full organics. If you use any chem/synthetic nutes you need to at least use water only a week before harvest.