Killing escorts is fun, but you'll get more tonnage saving your fish for another day. I'm happy knocking the largest taget in a convoy and slipping away until I find another group.
My crew also likes that I keep them alive and unharmed.
Plus it's more fun sneaking away while the DD's are thowing cans at me, it's the scenario that makes me love subs.
If your getting spotted at PD depth, keep the scope down. I think it's a bug but in SH4 you get spotted very easily if you keep perving on the enemy.