Just get all the relevant backlinks you can get, and if you are able to get some with high PR, then it's even better.
This is the best advice you'll get,google is very clever so it knows if your links are suitable for the page they are coming from.
EXAMPLE
A plumber who places his website in forum profiles in diy,plumbing building sites will have to do alot less work to rank high than if he placed his links in cooking forums or pet forums.
I'd go as far as to say relevancy is more important than pr.
You can always build up the PR of the link page by using scrapebox with,again,relevant sites with relevant comments aimed at the profile\link page.Maybe a bookmarking blast as well.
I prefer posting on forums,my links will be 100% relevant so my sig link means something I will search out a high pr page,google for an answer,spin it post and bookmark the page.Note this only works with relevant links if you place non relevant links it can actually damage your site because your link stands out like a sore thumb.So while this works for relevant links I have tested it with non relevant links and saw a drop,be warned.
Fact is the game has changed,automation is still very much alive and kicking but you need to use it smartly.
Another point to consider is only you start firing 30k+ xrumer blasts to get anywhere you must maintain this.These links will drop quickly and so will your site if you have say 20k links then drop to 12k your site is in trouble so you have to constantly blast highly relevant links means 2-500 will be the same as 25k of crap and the relevant links will stick.Pr isn't as big a factor as it once was.