Wrote off guide coats for 6 years, then a guy in Fresno made me eat it

I always figured guide coats were a crutch for people who could not read a panel by eye, so I never bothered with them. Last month a painter at the shop in Fresno handed me a rattle can of cheap black and asked me to block out a quarter panel I had already gone over twice. Halfway through the first pass the low spots showed up like a map, stuff I had been chasing with my hands for 6 years without ever really seeing it. I bought my own can the next day and now I hit every panel with it before I touch a sanding block. Anybody got a favorite color for this, or is flat black just the standard and I am overcomplicating it?