It's kind of hard to say. I personally never thought about it. I always look at John's style as "his own" and never judged it in the context of the time period he was living in. He was really hip with the way he dressed, anyway.
I'd probably think that the most "modern" pics are probably him around the late 70s, though not because its closest to our time. I thought he started to dress more "conservatively" as opposed to the white suits, plaid sweater vests, striped pants, etc. He was getting more into wearing nice casual clothes and jackets and having his shirt unbuttened a little, which is kind of sexy.
I also have a book where he wears some awesome sandals with a pair of khakis and a red shirt unbuttenened a little. Its pretty modern if you ask me. I'll try and find some pics, but probably not the specific ones I'm referring to.