Hi Brian!
I don't like to say "never" because each job is unique. I just encourage people to call or email and let us know what they've got and what's going wrong with it.
In most cases I leave English and other European single triggers alone. In any situation, the kiss of death is when an untrained but otherwise well meaning person has a go at fixing them. I've seen some real doozies. I'm totally biased obviously, but many factory single triggers just don't work all that well due to their designs and trying to fix them is a no win proposition.
Swapping out for a Miller is always a sure fix though!
Best, Dan