Adding it on bottling day could result in a lot more additional fermentation/carbonation/bottle bombs. I'll second what was said in the post before mine and vote that you add it at flame out to sterilize it some.
Reiterating the temperature & bacteria concern. Like already mentioned, flameout is a convenient time if you're OK with whatever the concentrate is being present for the complete fermentation. When I've dealt with adding adjuncts after fermentation has begun I prefer to go the method of waiting a few weeks for initial fermentation to pretty much run its course, then pasteruize the addition, cooling that slightly, and then re-rack the beer on top of the adjunct in a sanitized carboy. It seems to help with clarity and mouthfeel compared to adding earlier and/or not transferring.