Indeed it is an unspoken rule about not spinning the big wheel backwards, although a few times when contestants tried doing just that, Barker immediately stopped the proceedings and reprimanded the contestant for doing so.
As for Classic Concentration, it would drive me absolutely bonkers when the contestants would jibber jabber or worse, when Alex would jibber jabber for a while during the game. I even recall once a contestant snapped at Alex semi-playfully when he was asking "Can so and so remember where that match was? We just say it a moment ago" and she said something like "I could if you'd allow me to just pick already..." or something to that effect.
I suppose there were three logical points though with that going on...
First, indeed the show would be rather dry if they used the same method used during the 50s - 70s, just picking 2 numbers, the host saying "Not a match, the board goes back"...so to avoid monotony I suppose that's why the chattering was more prevalent on C.C.
Second, I suppose the show wanted to try to not have two huge potential payoffs during the rebus rounds since back in the late 80s, sometimes single game totals were even higher than the lower priced automobiles up for grabs during the Winner's Circle bonus game.
Third, if players were truly outstanding at concentration skills, no amount of jibber jabber would distract them from remembering where matches were on the board.
I for one whenever I play along always try to associate the number selected with the prize shown... (e.g. Luggage under # 14, I think "Luggage needed for a 14 day vacation", or "Refrigerator under #23, I associate 23 degrees being cold, freezing even, like a Refrigerator...of course that logic could work with all 25 numbers on the board!