My understanding that fit is even more key with DB compared to SB. I've been trying on RTW DB suits lately, trying to get a feel. Those that have fit well have felt awesome to try on.
Please, if you could elaborate a bit on what you mean by "need to understand what you like in terms of the style"?
In my case it was how big the "V" was where the lapels met - some jackets have the lapels crossing much higher up the chest than others and my preference is to show more of the shirt and tie.
My preference is for the latter. Both are valid but you need to understand what you yourself like.