A little over is better than bang on if you can cope with cutting the ends. You're more likely to get a square cut (because you care!) and more importantly you can float your joints around on the timber to try and avoid knots. The sawmill (in France so perhaps not the same working methods) took all my timber dimensions that I'd asked for (already rounded up, but in some cases, not by enough) and added a "bit". Generally this gave me enough leeway in layout, but for one or two it was a bit tight and I had to compromise.
