I find it's only really useful to know which gear you're in when you're coming to a stop, so you can be in a good gear to start off in again. For me that's normally 2nd gear at the back, middle ring (or sometimes outer ring). As the STIs will shift down several gears at a go if you push the lever further, as I'm coming to a junction I often just change down until it won't go any further and then change up one gear.
Occasionally I forget which front ring I'm on and accidentally change to the inner ring, which results in much frantic leg spinning as I start off...