The more sharps or flats the key has, the more the need for putting them in the key signature, not less. Otherwise there'd be sharps and flats everywhere. And you wouldn't be able to determine the key by looking at one bar.

Experienced musicians don't look at a key signature and say "okay, so I need to perform so-and-so particular substitutions", they look at a key signature and say "okay, so it's in the key of X".