I don't have specific rules on what assists I put on people, but draw back on ranged, pivot on armor, and reposition on fliers/melee is typically what I do. But for a team that's meant to work together I consider the whole group.
For example my horse team has 3 draw backs and swap on my heavy red (eldigan or xander). This allows my team to easily arrange itself into a square formation to get double buffs on everyone (I use double fortify/hone and two blade tomes). Swap is essential for this since as long as my characters are next to each other I can fix the 'buff square'.
There's also the aspect of constantly using new teams. I don't want to get mixed up and forget what assist is on someone, and end up drawing back instead of repositioning at the wrong moment. And when I fill in a unit for bonus, I like them to just slot easily into that role. So I do try to stick to my archetypes. Thankfully I seem to have developed a talent for finding unlimited Barsts so I've been able to do what I want lately.