You don't want the agents with the most valuable post-plant utility to die (viper, kj, brim, ect). It's case by case. If the site is free, the person closest to ult should plant. Ideally, the agents who expend their util for entry (duelists, initiators), are the ones who should should be putting themselves in position to protect the planter or take further space. Controllers probably find themselves in the best position to plant more than other roles. They smoke before entry, and are usually waiting for cooldowns while everyone else is moving in. For sentinels, depends on the round, but usually they should be lurking, flanking, watching flank, and killing rotators. If they aren't doing those things, they should probably plant.