My last bike was custom. I picked all the components including my saddle. My mtn bike was stock but I bought my saddle and had the shop install it when I had the bike fitted.
I won't put my saddles on any more, I have the shop where I bought my bike do it. They have the measurements and can put it on exactly where it should be. If I tried to do it I'd be adjusting and adjusting it. They just ordered a new one for me. I'm probably paying full price but it's worth it to me to have it put on correctly.