I created a new site and linked it to an established site of mine sitewide (100+ pages). The Home-page had a PR6 and the the sub-pages mostly PR5 and PR4. There was no difference in SERPS especially my ranking in Google which was on the 10th page. I decided to eliminate all the sub-page backlinks and only link to the main PR6 index page. 4 days later my listing in Google climbed upto the 3rd page. Big difference and I had not done anything else. There's only one way to find out - test!
I'll have to agree with this as well. Look at all the sitewides on http://googleblog.blogspot.com/ every single one of those links are being counted by google. So if they do not have a problem with doing it themselves then I would say it is ok to do so. However, I also think that is best that you do not go overboard with it. In other words do not make all of your links sitewides but try and spread them out a little bit.