The finer the line (some folks do a scribed line with a Very Sharp, pointed knife blade made just for this) the easier it is to know when to STOP with the plane or sanding block when you're doing the last bit of shaping.

Commonplace is to cut wide of the line by some amount (personal preference for value to swap for 'some' there) but I'd suggest 1/16" as a bare minimum.

Depends too on what you're using to do the cutting; Skilsaw demands real skill in control of that blade's path so 1/8" or more might be better here.

Japanese pull saws are popular 'cause you can steer 'em better with practice.

Powered jigsaws are kinda in-between... so choose your weapon & have at it.

What you leave between your kerfs & the plans lines come away slowly, with s Sharp block plane or other edged tools. Or sanding block.

Or maybe a belt sander if you're really brave....

So leave the inside of the line if you can, so you know your parts have the proper shapes for the jobs they have to do.

+1 What spclark said!

A sharp block plane makes shaving down to the line after you cut wide really painless.

the short answer, is that, unless told otherwise, it is not going to matter.  just try to be consistent in whatever approach you take.

more specific to your question, it depends on the drafting software that is used and if the thickness of the line was accounted for.  in most cases, it is not, so the line drawn is centered on the true measurement.   a typical plan line is 1/32 of an inch to 1mm...which is probably the line thickness on the plans you have... it is not going to make a material difference in your outcome which side of the line you choose.  fwiw, when i paste up plans to a piece of wood and cut them out, i typically shoot for my final cut/sanding to be touching the outside of the line.  the problem with the inside of the line, is you have typically erased your reference point...so you don't really know where you are.

to the question about technique, the guys above speak with some wisdom on the approach of cutting a bit proud of where you want to be if the tool is challenging to control and switching to another tool to make the final shape. i think when building from plans its worth the extra patience to get it "right" as it just tends to make everything from that point forward easier.   but i put "right" in quotes as you have some slop to work with without having to do an "do over" or discard a piece with minor imperfections.

h

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   Many thanks, guys. hspira, Yes. Your answer makes sense to me. If I go to the outside of the line then I can see where to stop! Thanks all.