So, to refute that point, first let's establish that, with the exception of walls, every time you spend life, you gain that much experience. Sometimes you can get experience for free, and walls are a loss on return and should always be broken last, anyway. In all other cases, you spend your HP, and if you don't die, you get that much in EXP back. Let's also understand that there is a max HP, but there is no max EXP, you can pool it as much as you want.
At the start of the game, you have 5 HP, and you need 4 EXP to level up. You spend your 5 life, and you now have 5 EXP. You now have the choice to either level up or to use a heal scroll. In the first case, your health refills back up to 5 (with half a heart towards a max of 6 next level!) and you have 1 EXP banked towards reaching the new limit of 5 EXP. If you use a health scroll, you refill your 5 hearts right away, and keep your 5 banked EXP.
Here's the crux of the matter. No matter which scenario you pick, you still have to spend 4 EXP to level up. You're not saving it for later, you STILL have to spend that 4 EXP, you're just delaying when you spend it. However, since there is a direct relationship between HP and EXP, you have assigned that scroll a worth of 5 EXP while using it while your max HP is only 5 hearts. If you waited until you had, say, 7 hearts or 10 hearts or even more, that is how much EXP that scroll would be worth. By using it early, you miss out on the potential for that scroll to be even more valuable. Meanwhile, the experience you gain by spending your health down as normal is experience you will gain one way or the other, and it will be spent on leveling up one way or the other. Scrolls are always worth more the longer you hold off of using them.
That said, unless you're aiming for something in specific, you don't have to be perfectly 100% optimal to win. It helps your odds, for sure, but if you would rather use a scroll for safety than take a gamble on something you're unsure on, its better to keep the run going than not.