Google Blog , Officially stated that “We encourage webmasters to use No Follow tag in there comment section to avoid Comment spamming”. you can read more here.So Google bot will never count No Follow tagged links in terms of Pagerank Juice.

Yet another reason to use No Follow is “Bad Neighborhood“.
If we give Do Follow link to any Bad site such as Casin0,P0rn etc, will harm your Pagerank and overall quality of site in terms of Google. and if our commentator will use such link then this may be harmfull for our site ranking only ..

So i populate few reasons or you can say few things,
“Where to use NO FOLLOW and where to use DO FOLLOW ?” in our blog.
These conditions are also applicable in your normal posts.

Where to use Do Follow
1. Related content sites.
2. Blogs who shares Pagerank Juice with others
3. High quality blogs (In view of Google e.g. Official Google blogs, etc)
4. Your Friends
5. From where your copied the post (If you copy something)

Where to use No Follow
1. Unrelated content sites. (Not related to your post topic)
2. Sites who never share there Pagerank Juice with other (Like Wikipedia, Youtube)
3. Your Feedburner Feeds (Why your Feeds need high Pagerank ?)
4. Casin0, P0rn etc like sites.
5. Affiliate Links (Why give them double profit ?)
6. Bad Neighborhood (Its you, who decide who is bad and Whos Good)

I will frequently update this post, if i got more reasons .

Look at this Image,
,

You have a wordpress blog ?
Want to make it “You comment , i will Follow” ?

Just follow these simple steps.
1. Either install any DO FOLLOW Plugin in your wordpress. which you can get just by Goooogling.

2. Or if you want to do manual tweaks in your files to achieve this then open the file “wp-includes/comment-template.php”
Now
(For WordPress 2.3 or older: line 48)
(For WordPress 2.5: line 114).
(For WordPress 2.6: line 116)

REPLACE :

$return = "<a href='$url' rel='external nofollow'>$author</a>";

with this:

$return = "<a href='$url' rel='external'>$author</a>";

Please observe this image carefully.

NOTE : Remember to take regular backups of your all files from server before doing any changes to files, i am not responsible for any side effect.

Now Done, You have hacked your wordpress to have DO Follow comment links !!
I recommend all blogger to use this tweak to give proper rewards to the people who comment on our blog !

* UPDATED : Currently my blog comments are NO FOLLOW, i actually suffer a lot from the comment spamming on my blog, so i decided to remove the DO FOLLOW from comment and makes to back to No Follow, your can get more information about comment spamming from here and here. actually i read those two posts and decided this . sorry guys.
You can read one of the main reason why i came to this conclusion here, Where nofollow and where dofollow