I would think it depends on each person's perceived image of an assassin. An assassin is simply someone who kills another for financial/political/etc gain. The method in which that person carries out the deed defines what type of assassin they are. If there are any Terry Pratchett fans out there, then think of Mister Teatime, who would probably have to have some strength to knock out the heavy set thug in the bar with one punch, yet was also extremely stealthy. On one of his missions he held a glass to the victim's mouth to make sure he had ceased breathing as was the customary method, but as Lord Downey pointed out, at this point the man's head was several feet from his body, which would require some strength.
A more agile, weaker assassin would rely on his other strengths, and cunning, perhaps using a poisoned blade before melting into the shadows.
It depends how you train your character at the end of the day. I don't think 37 strength is an unreasonable target though, provided your stats aren't completely skewed.
I suspect most or at least many thieves are heavy on agility/reflexes etc so it would mean that many would achieve the assassin title as a natural progression, which I think cheapens the title and associated image. It takes a different kind of dedication and training to achieve which makes it stand out. There are plenty of nice titles that we can/will get through the course of natural progression, those extra 'cool' ones need a bit more work which I think is a good thing.
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