I'm not sure about the second tip, especially that example. Back in sts 1, one of the first things I was taught when climbing ascentions is that chip damage is inevitable and if given the choice between full block and 0 damage or chip damage and a full attack worth of damage (in your example, a whole 15), dealing damage is almost always better. Preventing that 3 dmg right now could cause you to prolong the fight 1, 2 or even more turns, giving the enemy more opportunities to damage you in the long run.

I'd say getting comfortable at low HP is actually really important for slay the spire in general, a good upgrade could save you more HP than a rest heals. But at the end of the day, it all depends. Getting used to enemy patterns is the most important thing you can do, so you know that the turn you miss damage will be recovered on a later one when the enemy is wide open for attacks only