But you do see “cooked” resistors in old Fender circuits that would not have been damaged if higher wattage resistors had been installed in the first place. I have a great 1966-67 Fender Bandmaster head that was given to me by a friend because if was broken. It had one burned resistor, cost me like 6 cents to fix it. (BTW, I offered to give it back but he said to keep it. 
Using Ohm’s law to calculate the max wattage (P = E * I) and doubling it is usually safe.
The newer “Hot Rod” series of Fender amps uses 470r/5w resistors and although the resistors don’t fail, they run so hot that the wimpy wave-soldered connections do fail. Fender issued a service directive specifically about this problem and advises replacing the 470r resistors with 330r resistors (which run slightly cooler). You can see the discoloration (from heat) on the PC board in those amps.
Sometimes even 5 watts won’t be enough, if you have the room, there is no downside to using higher wattage resistors. I have a 20w resistor in the bias circuit of a 5E3 amp, overkill for sure, but it’s not gonna fail.