is it possible to add rel="nofollow" sitewide, so I won't bother adding rel="nofollow" for every link?
Why would you want to do that? Linking out can be very advantageous, it adds more context to your main content. In the case of Google, they analyze the content on the other side of that link in relation to your content. But to answer your question, yes there is a very easy way to accomplish that. Stick this in your HEAD section of any page that you want all links to be Nofollowed. <html> <head> <title>...</title> <META NAME="ROBOTS" CONTENT="INDEX, NOFOLLOW"> </head> Code (markup): Using the NOFOLLOW directive in the Robots META only applies to links on that page. It's entirely likely that a robot might find the same links on some other page without a NOFOLLOW. Note that the coding above allows your page to be indexed. If you do not want your page indexed, then change that to NOINDEX instead.
But When i analysed backlinks of one of the sites, it showed nofollow attribute to all other nofollow links but it did not display any nofollow attribute besides the link which were actually "external nofollow" when checked from the respective page source. Not sure if the tool was not coded to detect "external nofollow" attributes.
rel="external" = "This is an external link" (most are) rel="nofollow" = "Do not catalogue this link" rel="external nofollow" = "This is an external link, do not catalogue it."
Good for the user (slightly quicker load) Good for your bandwidth (a few bytes multiplied by a million over a few years might add up to something) Good for SEO? Less of the "same old stuff" means your template is slightly smaller than it would be, possibly reducing the likelyhood of a duplicate content penalty?