Hey Red,
One thing that I've found regarding commercially produced, pre-recorded tapes, is that the incidence of SSS seems to be quite low. Of the 500 or so, tapes that I've bought, I have 2 that have exhibited this problem. These 2 tapes were both produced by Ampex . If you collect a lot of later production Londons or Phillips this could be more of a problem for you.
Squealing tapes have usually been the result of some contaminant on the tape rather than from it. Quite often it's been caused by the glue from splices where something like scotch tape has been used instead of real splicing tape. The glue seeps out and onto your machines guide or head and that's why the squealing persists until you clean it off or it collects enough oxide to become less sticky.
The folks who really have problems with SSS are studios/record vaults and archivists. The huge popularity of Ampex 456 by professionals in the 70's-90's resulted in the need to bake these tapes for reissues on newer formats.
So, before you take out the oven mitts, be sure that SSS is what you've got. (there's a section on SSS and other tape maladies in the Beginner's Guide)