I personally just use a level 1.
Apparently according to threads I've come across in this forum, the biggest effect of the PR manager is reducing the decrease in attendance after you lose a big game (like losing to a rival or losing a TV game). But a quick look at the numbers makes me wonder if it's necessary - my game against the #2 team in the other conference had similar numbers to the #3 team in mine (and the former has a higher national rank). But I actually won the away game before the game against the #2 team, while I lost against my league rival (by 19 points too) just before the game against the #3 ranked team... The difference between both games was $3k (501k vs 498k). If I can achieve that with a level 1 staff, I don't think even bother getting anything more than that is necessary to combat the drop in revenue (I'm in Div I with a 22k pax arena btw).
Another effect I've read of is combating the loss of attendance when you relegate. I don't have numbers to back that up but if it's effective then you only need to hire one if you ever relegate.
That said, according to this thread, it seems that aside from specialty, it seems that it's not necessary to get anything higher than level 3:
(316991.1).
Still, my personal opinion is to just use level 1 unless you need one with specialty. If you do then it seems like you should go no higher than level 3.