I'm certainly no expert but I'll give it a go.
Fender didn't use screen resistors on a number of different models. Chronologically speaking the 5E_ class, if you will, appears to be the last overall class of Fender production amps that didn't use them. FWIW I think this was the case for a number of reasons including, but not limited to, the following;
1) Lower overall voltages tended to operate the screens in a more tolerable, if not tube safety conscious, manner.
2) In the late 1950s Tubes were plentiful and the general population had no reason to suspect that they wouldn't continue to be amnufactured and improved upon.
3) From a manufacturing standpoint it was cheaper not to use them.
4) No one had convinced Leo Fender that they should put them in until the 5E_A or the 5F_ series
Whatever the reasons were the lessons learned over the years probably shouldn't be ignored. As you have noted screen resistors are an important safety precaution designed to limit the amount of screen current and guard against possible tube failure resulting from too much screen dissipation and have an effect on overall tube life as well.
FWIW I used to use the absolute minimum value (i.e. 470 ohms like the old Fenders) but I have become a fan of the larger values/sizes such as 1K/5W in recent years even though I generally use lower power rail voltages than the 5E7 (for instance).
Perhaps someone else will step in with more theory than I can provide but; IMHO, they should be in there.