…Kinda like choosing a t-shirt; sure, you're not gonna be looking at yourself directly whilst wearing it, but you've gotta have liked it to begin with, regarding the style, size, colours etc.
Don't be like that. Have some free will. PLEASE put in some determinative feedback to YOUR potential specs.

…But not if you've been wearing specs for a while! And if your retort is "well, i'm not going to be seeing myself"; true, BUT 1) others will, and will judge YOUR final decision, 2) you must surely have some degree of free will, and it's YOUR purchase, so you kinda have to like what you wear…

…Then you are a prime example of what the Germans refer to ask "backpfeifengesicht" (a face in need of a fist).
Yes, dispensers are there to helping you choose new eyewear, and "I don't know" is a perfectly acceptable answer if it's your first time choosing/wearing specs…