When should one make exceptions in iambic pentameter/meter in general?

Answer 1

If it's for a school assignment, the teacher wants to know that you understand the verse and meter of iambic pentameter. But, I would definitely suggest quality over form in any form poem. If you have a word that makes a stronger impact - whether that's in the poem's overall meaning, detail, rhyme, etc. go ahead and use it, but I wouldn't go over or under by more than 1 syllable.