Based on what I have read, that could reach 10 degrees higher....which would put me at much higher than suggested temps.
I keep hearing that 10 degrees, and sometimes it is 5 degrees, but it always anecdotal. I suppose at some point I will set up my own experiment, but with all the agitation going on in the fermentor from thermal gradients and CO2 production, during active fermentation I can't see how there is going to be significant differences in temperatures anywhere in the beer. The difference in temperature between a thermowell and something on the outside of the fermentor will strongly depend upon the type of device and how it is mounted. A hard cylindrical temperature probe (like from a Ranco controller) makes horrible physical contact (almost no surface area contact) with the cylindrical wall of the fermentor, so it is important to cover them well with an insulator (bubble wrap) and lots of tape because the probe will measure the temperature of the air around it better than it will measure the temperature of the fermentor. A stick-on thermometer makes great contact with the fermentor via its adhesive, but you're relying on something that has one side exposed to the air and the other side that has a thermal insulator (glass/plastic) between it and a large thermal mass (beer).
As Sean says, if you really want to know, you need to calibrate them, but make sure you calibrate them like you will use them because you might find the stick-on ones have one offset when on a glass carboy, and another when on a plastic carboy or bucket. The good thing about all of this is that if you set everything up the same, you'll get consistent results. You need to keep in mind that if someone says they get great results fermenting at 64F, not knowing how they are measuring temperature and what their offsets are, that might not be the same 64F to you. (Yeah, I know, I hate those "it is different from system to system" answers too...)