I'll have 50c each way on this... depending on how many pages are in the blog.
IF you have hundreds of pages in your blog/site/whatever, then a new link in the global navigation could be seen as link spam by the search engines.
e.g. I linked to one site from the global navigation at the bottom of 5000 pages on another site recently (as part of an
SEO contest) and noticed my position in google results plummet very quickly. It took weeks to fix.
If there are only a few pages in the blog, then by all means put the link in the nav bar... It's there for convenience to your visitors if they wish to explore further.
Remember to TEST anything you do like this, and watch Google closely over the next few days.
I'd definitely put the link in place with the actual post content. It's in context there, and people with an interest in that article/item are more likely to click through and take action at the other end.
Cheers
Stephen