No it does not. Infact every little helps. You should exchange links with related sites for a maximum effect though.
so if I have a 20 websites with PR1 and you'll have 1 website with PR5 and if i'll link to you from my 20 websites ... your PR will be decrease ?? think just a little
Just gain up backlinks, doesnt matter of PR, it just helps a little like the others said, you can have 1000 pr0 backlinks which can be same as 100 pr2s, just a little example