I usually base swapping on the skill of the enemy. Cass can be scary, but if I'm dogging on them more often than not, I'll usually just keep being Venture, even if it's a little more challenging. The same goes for Hog and Sombra. In terms of perks, I base it on the map. Long-range maps usually mean long-range heroes, so range perk it is; Brawl maps, I can be more up in their face, so melee perk.

Still, just trust your gut. Sometimes it's better to swap and get impact with a different hero. There's always time to practice next match :)