Gauge swatch question

[sueko]

I have always avoided patterns that required a gauge but now find myself in the position of wanting to make a sweater that actually fits.

My question is, when doing a swatch, do you wash and dry and block it before measuring, or do you just finish the swatch and start measuring? For that matter, do you pin it down on something like an ironing board, and how accurate do your measurements have to be? When do you know it doesn't make the gauge? 1/2 a stitch? a whole stitch? 1/4 stitch? Just how finicky is this gauge swatch business?

Thanks bunches