Interesting question! In most smaller lakes trout use the whole lake at various times. Some of the use is for comfort(as when they are sitting on the 22-25' thermocline) and some of it is for travel to and fro.
A better question is" Where do trout eat in lakes"? The answer is the Littoral Area. That area generally between 3' and 15' deep that grows the vegetation necessary to support a food chain.
In clear lakes it may go even deeper to 20' and in high plankton,murky lakes it may end shallower.

I see many fishermen casting for the middle of the lake with no idea whether fish are in that 30' water or not,just a general feeling that the deeper water holds the most & biggest fish.
WRONG!

Fish the Littoral Area where there is the most evidence of fish or fish food (insects and nymphs)
showing.