Again, experience here differs.
I've had situations where - at full pop both times - they meet six wandering orcs and sail through without losing a single hit point, then meet six more the next day and almost TPK.
In fact a version of this became a long-running joke/meme in a previous campaign of mine: whenever the party stopped off in town for some downtime, their first combat of any kind back in the field - no matter how easy it looked on paper - would be a complete crap-show! Six nobody bandits once nearly TPKed a 4th-5th level party of 7 characters who collectively shouldn't have taken a scratch; but the first-combat-out jinx struck again and they had to stagger back to town to recover.
Not me, other than to the vaguest sense of level-appropriateness that any module has. The adventure is what it is; and if they decide to take a party of composition A in there as opposed to another of composition B (or size A or B - party size can vary quite a lot as well) it's not going to make me change anything.
If an adventure demands they have a mage in the party, for example, and they didn't bring one (or the one they did bring is dead) then when they get to the point where the mage is essential they're gonna grind to a halt. In cases like this, if they think to do any info-gathering ahead of time I'll try to work in a hint somewhere that having a mage and keeping it upright would be useful; but if they don't ask, so be it.
If the Big McGuffin found in the adventure can only be used by a Good Cleric and the only Cleric in the team is Neutral, then so be it: they don't get any use out of ol' Big McG.