Actually, in the core Marvel Universe at least, Strength has nothing to do with it, only being deemed worthy. There are a handful of exceptions to this, usually being either 'what if' stories or a different universe and thus different rules. The Rogue example, for instance, was from a 'what if' where she ended up absorbing Thors mind and powers completely, ending up as the new Goddess of Thunder. The Magneto instance was in Ultimatium, part of the Ultimate Universe, where the writer simply ignored the established rules of the canon to make Mageto seem more badass. It was the least of the writers crimes in that piece of crap. The same writer was the one that let Red Hulk use the hammer once, trying to explain it as 'they were in space, so Mjolnir didn't know which way down was'. That was during a storyline where Red Hulk was so amazing he beat up the SIlver Surfer and nearly killed Galactus because his name was Marty Stu. That was declared non-canon FAST.
Cap and Superman being worthy of the hammer is canon, as is perhaps the best case. After a fight, Thor sat down to catch his breath, and a paramedic on the scene spotted Mjnolnir lying nearby, and picked it up and returned it. Thor took it, blinked, double-taked, but before he even saw the guys face, he'd vanished back into the crowd to work
Cap and Superman being worthy of the hammer is canon, as is perhaps the best case. After a fight, Thor sat down to catch his breath, and a paramedic on the scene spotted Mjnolnir lying nearby, and picked it up and returned it. Thor took it, blinked, double-taked, but before he even saw the guys face, he'd vanished back into the crowd to work