A psychological observation (kind of)
Please, don't laugh... Look, perhaps this is common knowledge/trivial info or, on the contrary, a totally incorrect assumption based on games vs. unskilled noobs/mids. Anyway, I think that during end-game, when a lot of action is going on, and preferably on 1v1/2v2 settings, sometimes it's better to just "neutralize" opponent's non-crucial flags (not portals or gold, of course) than to actually capture them. The reason is ridiculously simple: "neutral flag" color closely resembles a regular "allied flag" green. If the opponent is not 100% map-aware (and during end-game he has a lot to worry about apart from closely contemplating that mini-map), the chance is he will notice something is wrong way later than he would have I opted to capture the flag (= turn it red). It is not a purely theoretical observation, and it works like charm in 1v1 vs mids (to the point I am actually able to shut down an enemy Valor flag on Prison quite consistently for most end-games). The same can be valid in 2v2 (especially on larger maps like Leviathan). However, I'm assured in any standard Cataract 3v3 even a half-decent non-coordinated team w/o voice chat will notice it quite quickly (statistically, there is a huge chance one of the three opponents will notice it), so it's not really worth the trouble.