In a lot of amigurumi patterns, you'll probably see a note like this: "Gauge isn't important, adjust hook size as needed to achieve tight stitches so stuffing doesn't show through."
I think even in amigurumi, gauge must be considered. If you look at some amigurumi projects on Ravelry,
quite a few of them look completely different from the
designer's in proportions and overall size. And then the designer gets comments about how it's a "bad" pattern but it's not always the designer's fault.
Even if you use the hook size and yarn weight specified in the pattern, tension still varies from person to person.
Getting tension just right
takes time and practice. You should make swatches in rows and
in the round to see how your tension is. Just crochet how you normally do
it. Then make swatches in looser and tighter tensions so you can see the differences.
If you become more familiar with your
tension, it makes it easier for you to modify patterns accordingly if you find that your pieces are coming out smaller or bigger because of your tension, hook size, and or yarn weight.
Tension can also vary
for you at times. If you are stressed, you are more likely to crochet
too tightly. If you're happy and relaxed, your tension will be
just right or even a little too loose!
When I was first learning how to make amigurumi, I had a lot of trouble getting it right. I would use F
and G hooks because it was the size that most people used but I kept getting holes. I crocheted too loose... too tight. I decided to try B and C hooks to get a tighter gauge. But after a while, the small sizes hurt my wrists too much so I went up to E hook, then F hook, and now... I can finally use G hook comfortably. :)