I recently heard a SEO guy claiming that google overlooks your "good quality links" if most of your links are of low quality, is this true? He claims that google after seeing a few bad links to your site decides to somehow label the rest of your links as being of the same quality.
I doubt that's true. Google has made it explicit that who links to you can't hurt you. If it were people could just link to their competitors from low-trust sites, and get them out of the game. A trusted link from wikipedia or dmoz is not going to get spoiled by some adult site also linking to your site.
I used to think that sites linking to you couldn't hurt you but since then I have seen stuff that makes me think otherwise. One case is the TrustRank algorithm which actually can discount some of your links by applying a negative factor in the ranking equation. This sounds like what this guy might have been getting at. The thing is, you have to have a lot of spammy links to offset good ones and, if you are well-established, there's no way someone could take you down. Like no matter what you do, you couldn't hurt wikipedia for instance but if you have only a few good links and a bunch of spammy links, the TrustRank algorithm implies that it can discount your good ones.
I think if you stay clean yourself (not link to non-trusted sites) and link to a few trusted ones, a few spammy links your way can't hurt you. Certainly won't get you trust either though - that's pretty clear. But it's true, I haven't tried this or anything.