I go by audio and always use quality headphones. I decide by audio to dig or not. Then after that, I look at the display (providing I'm using a display screen machine), digging or passing. I hate to get a hint of a sweet sound and walk away, so now dig so I won't think later "should I have dug".
Sometimes I get a little good sound on a deep target but the display don't respond to it.
I use the the display secondary, but have pretty much made my mind to dig or not via audio. Yes after deciding via audio, I still look at the display afterwards just because its there and I want to see what it shows. So I don't depend on the display - just use as a secondary tool I guess.
My first 3 or 4 detectors did not have a display screen. Then I bought a new machine, Whites 6000 Di Pro Series 2. It had a large analog meter with color scaling. Wow, I liked it but was so used to headphones and audio. Later, years later, I started getting digital display machines. Now I have machines with and without display's. No matter, I like them all for one reason or another. But my audio only machines (i.e. Tesoro's), I feel fine using them.
If I get into an area and hit good on common coins, then the display can be nice ID'ing the coin (VDI) and depth and that kind of adds a nice touch at times. But I won't feel all handicap using a audio only machine, even a single tone machine. Kind of like it both ways I guess, reason I have both types.