HI Jubaer,
Last year, Cyrus Shepard did a White Board Friday and touched on this very subject.
REF: https://moz.com/blog/the-rules-of-link-building-whiteboard-friday
Quote: "...
Don't link externally in the footer
A couple of other rules that I see people violate all the time that Google has made painfully clear in the past few months: Don't link externally in the footer. Just don't. I'm not going to go into the reasons. Just don't do that.
Avoid site-wide links
By the same token, except for navigation, avoid site-wide links. This is something that we've known for years. If someone links to you externally, site-wide, in the side bar, that's ripe for Penguin-style links...."
The idea of linking to your site on every page is an old tactic used to generate higher DA / PA for the linked site. There is of course something to be said about a company that is proud of its work and is happy to put their name on it, but in general the idea of harvesting clients link juice in an effort to boost yours is a bit pretentious.
At best you get credit for 1 link on the site, at worst you move the bar towards getting a penalty for spamming.
My advice, if your clients are okay with it, pick a page like "about us" or "contacts" and put a small blurb, site powered By "WordPress" designed by YourDesignSite.com. This would be 2 external links and draw a correlation for scrapers between the CMS, the Keyword Design(ed) and Your site.
Hope this makes sense and helps,
Don