Take a look at some loading data that has multiple data points. For example, the free Hodgdon Basic Reloaders Manual 2003, page 51, shows the following using International, the Win 209 primer, STS hulls, Rem Figure 8 wad, and 1-1/8 ounces shot:
17.5 grs/1090 fps/7,900 psi
18.6 grs/1145 fps/9,000 psi
19.7 grs/1200 fps/10,200 psi
20.8 grs/1255 fps/11,300 psi
If you were to plot this and use, say, the 1200 fps point, a 0.3 gr. variation around the 19.7 gr. mark would give about 1180 fps at -0.3 gr. and about 1220 fps at +0.3 gr. As for pressure, the corresponding values would be about 9,900 psi at -0.3 gr. and about 10,500 psi at +0.3 gr. In other words, one would be looking at about a +/- 20 fps variance about the nominal 1200 fps and about a +/- 300 psi variance about the nominal 10,200 psi.
Is this significant? Probably not with this particular load. It could potentially become significant, though, if one were out there on the high edge of the pressure curve. (SAAMI maximum average peak pressure for the 12 gauge 2-3/4" and 3" is 11,500 psi.) Significant not in the sense of vaporizing your shotgun, but simply adding unnecessary stress to the action and perhaps blowing the pattern.