I don't do it every session; just when I want to challenge the players. The rest of the time, they just don't get the 6-8 encounters, and that's okay.
I wonder how you challenge your players in low encounter scenarios then. Do you not rely on the resource mechanics at all in that case? To me that feels like throwing a big part of the D&D design out of the window. Which isn't necessarily a bad thing, it's just becoming a different sort of game at that point.