@russkil I think @dankcushions said it best. It just depends on the build, case, and how you run your setup.
I run a Pi3 without overclocking, and for the arcade games that I wanted to play, I was easily able to push it over the threshold and saw performance suffer as a result.
I cut a square off of an old Pentium sink and polished it smooth, then stuck it onto my CPU using Arctic Silver. No fan. I have never seen a temperature related issue since, so I will do this on all of my Pi3 systems from now on.
I echo the comments above regarding self-adhesive applications. They can work if the stuff is thermally conductive, but how can you know what you are getting? Having a tiny dab of actual thermal adhesive is a good fallback.
Regarding paste vs. adhesive, both will work. The adhesive hardens while the paste remains liquid, but the paste is thick stuff and it dries a bit with hot/cool cycles and it is very sticky (don't over apply). Mine sink has not moved a bit since I added it (and the Pi is attached upside-down inside my cabinet), but I could grab the sink it and twist it right off if I wanted to--so thermal adhesive is probably the safer route.