i always wear shorts and a tshirt when doing estimates with sneakers. i hate flip flops.
i never do estimates while i'm out working. i wait until i'm done working and then go. that has only happened a few times in which case i just wore my same work clothes that i had been wearing all day.
most of the time people call for an estimate on saturday. i don't work on saturday so i just wear my normal everyday clothes to go do the estimate. i don't dress up for it.