I've been using Hemrajani for awhile, so I go and look at swatches for the most part. Most of my purchases were rational, "I have a hole in my wardrobe and could use this" sort of things. Between having a decent assortment of tan and grey trousers, I wasn't super worried about matching most things with trousers. I usually try to be pretty objective about what new items I could use and come in with a sense of what I want. I'll also look at some other swatches and file them away for future purchases if I decide I could use a jacket like that after thinking it over for a couple months.
My two big mistakes were a grey herringbone sport coat (one of the first two things I bought when revamping my wardrobe a few years ago) and a black and white houndstooth with a cashmere-esque feel (bought via impulse via email after seeing the swatch elsewhere in person). I like to think I've learned from these, so now I try to be pretty targeted and think things over a lot beforehand.