The problem with songs that have "obvious" chords, say a three-chord blues in A like Rock Me Baby, may not be that the chords aren't clear (using the I-IV-V guideline, we may know that the other chords are D and E), it may be that the progression is in doubt. The only real difference in playing Rock Me and Key to the Highway in A is the sequence...A-D-E for Rock Me, A-E-D for Key. Sounds like simple stuff, but I have seen good guitarists fall on their face when presented with the simple instruction "this is a blues in A". That's why I like to write it down, anyway. And it stinks when you play Key to the Highway and the flute player and fiddler at the jam go into their solo breaks with A-D-E assumed and change the song entirely.