For boss fights, backbar. The single target damage from infused prismatic is more than the weapon damage will give you.
So Rakkat, Lors Falgravn, some AA bosses etc you want a Infused Prismatic backbar for the most damage.

For trash fights with lots of adds, backbar it can only hit one of them each time, so would be less damage output than having Weapon Damage glyph backbar which boosts damage of all skills.
In this scenario you can use put it on front bar instead of flame glyph for a bit more damage.

In PVE, its typically used on the back bar instead of a weapon damage glyph for certain boss fights. There are probably trash pulls that it makes sense to front bar it, but I honestly dont think its worth the hassle. You dont back bar in trash because it doesn't hit multiple people, but weapon/spell damage will boost all abilities. My main trial DPS always has two VMA weapons, one with weapon damage and one with prismatic. If going for scores or just fun boss parses, it is worth the swap, but not by a huge amount.

In PVP, I would say it is generally used on the front bar. There is a lot more build diversity in PVP, but usually I use my front bar for damage enchants and my back bar for buff/utility enchants. In PVP, I dont run any type of weapon ground dots (blockade, hail) that is going to reliably proc my back bar enchants, so truthfully, your back bar enchant is not nearly as important in PVP as it is in PVE, because in PVE, it will be always procing on cooldown. I usually just go back bar weapon damage glyph that I try to proc during my burst combos. In PVP, prismatic is strong vs some opponents and useless towards others, so its judgement call as to whether you want to run one at all.