If I'm making a few small profitable trades every day, how much should I actually care about fees and spreads? They seem tiny compared with the amount I'm trading.

That's what I thought at first too. The problem is that you're paying those small costs over and over again. Spread, fees and slippage can start eating into the result when you're doing a lot of round trips.  I think the easiest way to see it is to calculate your actual monthly trading costs instead of looking at each individual trade. This covers the hidden costs pretty well: https://mantelligence.com/three-findings-before-starting-day-trading/.