Firstly, Roger Musson advises rounding down, rather than up. My feeling is that spokes 1mm too short is quite ok, 2mm too short is kind of outer limit and 3mm too short is going to be too weak (i.e., not enough threads will correspond). Spokes too long is similar in terms of strength, but a pain in terms of driving with a screwdriver.
Someone will probably come on and say 'I never bother and some of my spokes are 5mm too short and I've never stripped a thread yet', in which case good luck, but I doubt whether the spoke makers would go the expense of making spokes in 1mm increments if any old length would do.
I think you can compensate for shorter spokes if you use longer nipples with more thread, but I'll have to leave it to somebody else to explain that, as I've never done it.