I've not been to the Prague houses, but I've been to most of the other Central and Western European houses. New production premiers are sometimes dressier than other nights (example Munich and Vienna), and some theaters have local rules (for example, the poltrona (orchestra seats) at La Scala).

With all that said, I never drag a suit along on my trips and have never felt out of place (my usual "uniform" is dark wool/blend slacks and solid black shirt). The older peope (50+) tend to dress up more, but you'll find all levels of dress across different age groups.

Obvious exceptions are places where dressing up is part of the experience, such as Glyndebourne.