I disagree, not making it explicit adds a level of abstraction and assumes that everyone needs to know how online affiliate programs work. Users should know, without any question or confusion, who exactly is benefiting and how whenever they use an affiliate link. Otherwise there are perverse incentives.

And its not just me, here is the FTC's stance.

You should disclose your relationship to the retailer clearly and conspicuously on your site, so readers can decide how much weight to give your endorsement. You could say something like, “I get commissions for purchases made through links in this post.” In some instances – like when the affiliate link is embedded in your product review – a single disclosure may be adequate.

I know FTC is American, but I think its a good guideline.