If you’re knitting a jumper/sweater a tension square is the most important part (I don’t think I’m exaggerating here, it’s pretty important).
(1) Tension impacts the size of the finished garment - too tight and your garment will be too small, too loose and it will be too big and you might run out of yarn. You’re knitting, so really this isn’t a big deal but also, is it the worst thing to slow down and save some trouble later?
(2) Depending on what it is you’re knitting you can practice - what is the stitch pattern like? Are there any places you might go wrong? What do the colours for colour work look like together? Particularly if you are knitting colour work, lace or cables blocking is essential in this respect. See below.
(3) While after a while you can generally guess a little bit, for example, I knit tight so generally my initial tension squares have more stitches and rows than the instructions state, different yarns from different varieties of sheep/yarns with different compositions (nylon/alpaca/acrylic etc) will knit up differently, as will different weights of yarn.
But then again, tension squares take time and yarn and what will you do with the square later?*
Best practice is to knit a big tension square, at the minimum a little larger than 10cm and then block it, that is, soak it in water and then lay it out, pin it into position and then wait for it to dry/ encourage it along with a hairdryer/heater. If you’re organised and you do this the night before, you’re winning.
But hey! We’re knitting socks and we generally knit socks in the round and so the rules change. You should knit your tension square in the round. Ysolda also has some great instructions about this - https://ysolda.com/blogs/journal/swift-swatching-in-the-round
The biggest issue I have with this is that if you cut your tension square it makes the yarn unusable in the future. This is great and vital if you’re making a jumper, less great if you have one skein of yarn to make a pair of socks. I have definitely been known to unravel a tension square when I run out of yarn as I get to the end of a project.
Again though, we’re knitting socks, so if you’re not too worried about time/starting over it is acceptable to take a guess, start knitting and then check halfway down the leg. This of course depends on whether the yarn is likely to shrink or grow when washed and how on the edge you want to play things. (Personally, I’m going to knit a teensy tiny square, really more of a teensy tiny rectangle).
Next time I’m going to post a picture of how you read/measure a tension square and what to do if it isn’t the right size.
* I wish I was the kind of person who knit the same size square every time and then made a freaking awesome square blanket. I am unfortunately not that person.