I did a swatch for the tank I am going to knit. I had the guage absolutely perfect before I blocked the swatch. I am using 100% peruvian highland wool ( Wool of the Andes) by knit picks. The pattern is Butter by Melini Melini Designs. The pattern calls for 6 stitches per inch and 7 and 1/2 rows per inch. After I blocked it I am still right on for the 7 and 1/2 row per inch but the stitches are only 5.5 per inch. that 1/2 a stitch difference per inch seems extremely minimal to me as it means I would be out less than one millimetre per inch. But when I figure it out over all with casting on 180 stitches it would mean that I would be about 15 stitches too big after blocking.
Now if I go to a bigger needle will it wreck the 7 and 1/2 rows per inch? I am not sure if this part matters so much as it says stitches per inch are more important to fit correctly.