Harlequin

Can depend on the type of person, but likely for most ideally a bit of both.  Plan, but be willing to change when laying things down don't look or feel right.

Yes, there are a spectrum of possible approaches.

In an active current thread (mdvle knows which one) a member is being given the whole range of advice from measure accurately and plan on a computer, through pencil and paper, to building the boards first and only then think about what goes on them!

I tend towards the planning end of the spectrum - and I recognise that one of my failings is that I plan a bit too much before actually doing anything.

I guess to be a bit clearer, I think the issue is that there are some people who just have the ability to visualize in their mind a layout given an in place benchwork and just start laying track.  Most of us don't have that advantage so the time spend planning is a necessary part of the process if for no other reason than to reduce waste as we discover what we want doesn't fit.

It also will depend on the size of the layout.  It is one thing to grab a 2'x6' piece of wood and play around with some track, another thing entirely when you are looking at the cost of wood and other materials for a 20'x30' space, where having a design to try and follow can help save money on materials.