I've never really paid too much attention to what gears I'm in other than trying to keep from cross chaining. My Giant Roam 2 has indicators, but I've always looked down to get a general idea. My street friendly MTB has them, but they don't work. My 2 road bikes don't have them.
I usually stay in the middle chain ring regardless of bike and use the rear to go up or down. It's pretty flat where I live so it's not been much of an inconvenience.
Occasionally I'll play around with different gear combinations in an attempt to ride faster, however, most of the time it seems like that tends to lower my cadence too much.