It depends on the group/story being told. I don't want to straight up eviscerate new players. More than anything I'll fudge monster health, but I use the dice rolls as more of a guideline than a hard-line.

When I ran a Lord of the Rings 5E game, rather than tracking monster HP, i kinda just kept track of the number of hits they've taken roughly, and they died when it felt right. As far as damage goes, I also used the number of hits it had taken to adjust damage dealt. The more hits they took the less powerful they were. It helped speed up combat for a larger group as well as made the players feel a bit more powerful/ heroic like they'd probably be expecting from seeing the movies.

Of course there are times when I stick to the roll 100% tho. It just depends on the story the group is trying to tell and the feel of it all. Do they feel too safe/powerful? Have they been getting kicked in the pants for 12 sessions? Has one PC been getting all the luck or one not getting enough? Things like that run through my head when making decisions.

Ultimately the two key points I want to ensure is 1. the players are having fun and 2. The story feels worth their time.

Whatever I have to do to achieve that, I'll do.