Of course, top pair/top kicker is often still the best hand, but nonetheless it will be more likely now a) that you are already beat and b) that your opponent has more outs.
As a rule of thumb, you shouldn’t move all-in anymore with a top pair on the flop and you usually shouldn’t bet it for value anymore on the river.
With a 36 card deck, there are simply more higher value hands possible such as two pair and straights occur in almost every hand especially if it’s multi-way. This being said, your top pair will be stronger if you are heads up, opposed to when its three or four way to the flop.
You will have to pay attention to the boards run-out if you are going for value with top pair on the river.
Let’s have a little Quiz to find out in which scenario we should go for value on the river, considering all hands are a heads up pot.
Hero has AQ and there are no flushes possible.
Board 1: A-9-T-9-7
Board 2: A-J-9-6-T
Board 3: A-9-J-8-K