[iconoclast]

The general rule for ANY adverbial inserted into the verb phrase is
(1) Adverbial + One-Word Main Verb, or
(2) auxiliary + Adverbial + (auxiliary) + (auxiliary) + Main Verb,
with the present and past simple of 'be' the exception, e.g.
1. She's always on time.
2. They only go to church when his parents visit.
3. I think I've just broken my toe.
4. He doesn't always keep his promises.
5. They've never been stopped by the police.
6. They can hardly have been given the right information.

There also seems to be a little flexibility in some situations, indicating small shifts in focus, e.g.
7a. This could easily have been avoided.
7b. This could have easily been avoided.
7c. This could have been easily avoided.

Traditionally (in British English), (some) frequency adverbs move in front of the first auxiliary or the verb 'be' to indicate stress, e.g.
8. He NEVER is late.
9. She ALWAYS has been a churchgoer.

They can be stressed in normal position, as well, of course, e.g.
10. He's ALWAYS late.
11. She's ALWAYS been a churchgoer.

The adverbial 'never' can front a clause for greater stress (part of the pattern of negative fronting involving inversion, e.g.
12. NEVER have I met such a bunch of idiots.

However, in the American press it's common to see traditional British stressed formation with no stress intended.