It's always tempting to earn an extra buck. As a freelancer with a growing client base, you'll run into people who want you to insert links. But should you?
At one point in my career, I wrote for more than 20 blogs. As a result, a number of marketing types asked me regularly if I would insert links into posts on these sites. In many cases, I was offered as much as $500 to sneak links into blog posts.
I never took the money.
My policy is to avoid getting paid twice for the same content, and especially to not take advantage of my clients — especially since many of my long-time clients are people I know in real life and are people I consider friends. The last thing I want to do is jeopardize my reputation and my relationships with some sort of scheme to insert links.
Remember: your clients own their blogs. They should be able to decide what is on their sites. While I sometimes link to high-quality articles on other sites, or stuff I've written elsewhere, it's always done after getting permission from blog owners who trust me to not take advantage of their link juice.
Only you know what's considered ethical for you, and only you can make this choice. But you should really think about how you want people to view you — and what it could do to your reputation if clients find out you're double-dipping.