Here's what I would do for a relatively light cabinetry or woodworking project.
First, don't plane a board if you aren't going to use all of it at the same thickness. It doesn't make sense to plane a 8' board if you are only using 4'. (Always observe your planer's minimum length requirement.)
My general principle is that I want to power plane the wood as long and thick as possible, but there are some good reasons to cut to rough length first. For instance:
* Evaluate how wonky the wood is. If it is relatively straight - e.g. minimal bow, twist, and cupping, go ahead and plane first. If you can't get the thickness you want without cutting to length, then cut to rough length (finish + snipe length) to get more thickness, and then plane.
* Do you need to work around defects that you don't want in your project? Since I have about 1000 feet of #3 common cherry in my basement, I do this a lot. If you are using FAS, or if you want to integrate the "defects" into your project, you don't have to worry about this step. Bear in mind that in woodworking, one man's defect is another man's feature.
After you get the pieces you want planed to thickness, then do all of the final machining - ripping, cutting to length, etc. You can sand or (hand) plane smooth as needed after that - exactly when often depends on what the assembly looks like. I like to smooth as much after assembly as possible, but sometimes you have to do it before to get at corners, tight spaces, etc. For example, I always smooth a table top before assembling it to the base.
My caveat - this fits my tools and the kinds of projects that I do. Other patterns of work may be a better fit for your projects or tools. For example, a professional cabinet maker would probably work very differently - an efficient, fast processes, specialized tools, and no #3 common.