Do modern people have to follow the strict relationship between the level and the oblique when writing a stanza?

Moderns and ancients are definitely not criteria for dividing poetic forms

A strict jiejie is jiejie, and its criteria don't change, and there's no modern-ancient distinction

Your question suggests that people nowadays are more casual in their writing, and there's nothing prescriptive about it, but

It's not jiejie if you don't follow the criteria

If you don't follow the criteria, it's not jiejie.