Do you consider 0.5mm (1/50") off significant?

[Cabinet Monkey]

For woodwork - no.

For making a watch movement - Yes

[6270_Productions]

Wood moves with humidity changes. 

Very carefully measure and cut your board to the exact dimension you need.  Use a marking knife, if you want a really thin line . . . then set it on the back deck - in the sun - for the afternoon and leave it there.  Measure it after 4 or 5 hours and get back to me.

That being said.  I use a mechanical pencil in the Shop with a .5mm lead.  I cut to one half of the line, when I measure and then make the cut. 

No, I don't know how much .25mm is.  Those are the kinds of numbers I measure with micrometers when using my metal lathe.