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Heavens don't be ashamed to make a gauge swatch! If you want a garment to fit you really need to do it. I always use them for my own sweaters, but I don't bother for hats or baby clothing since that isn't as critical.
And yes, changing needle size is very common. Some people knit looser or tighter than the designer of the pattern or the yarn. I knit loose myself so I often have to go down a needle size.
BTW..I don't know what yarn you are using, but you should also wash the swatch the same way you'll be washing the sweater. That way if it shrinks or stretches you'll know ahead of time and can adjust accordingly.