Hi I am a web designer, so on every page of every website i build i have a backlink. These websites are obviously never going to be in my niche. So surely this will hurt my seo to have many inbound links from random niches. So my question is, how much effect does the niche of a backlink have on general seo scores etc. and what do other web designers do on this situation? Is it better not to have a link there? make it nofollow? etc Thanks
Google algorithms are mainly focused on relevancy and popularity of a link/page to index and rank a website! So if you would like to get search engine rankings you will need to focus on these two factors! No follow attribute is applied not to follow any ranking to the links residing on the page! If you think your webpage has the links from different niche and you don't want to completely remove them, then no-follow will work out for you!
I would keep them there. Even though they will be mostly unrelated, they are still backlinks which MAY give you a little PR juice. Do a search for something like "website design", then have a look at the backlinks Google shows for the top couple of sites, you will see that they are mainly from their thematically unrelated customers. The fact that google shows them, means they must recognise them and they are a big part of the reason those sites are showing exactly where they are in the SERP's. You could experiment with just having your link on their HP, as unrelated sitewide footer links, have been known to be discounted. So it MAY be that just one link would actually be of more benefit, but as I said you will need to experiment with this to really know for sure.
You will not get penalized for an irrelevant backlink. Worse case scenario it does nothing for you. Best case scenario is the page gains some authority and passes a little link juice to you.