Regardless of whether I'm wearing a suit or sports jacket, tie or no tie/ unbuttoned collar, I always wear a pocket square.
(FWIW, I wear more odd jackets and slacks than suits because I like to mix it up a little and throw in some extra colours or patterns)
In the rare event that I might forget to wear one, I feel like something is missing. As if I left home with only one sock on^^
Getting into pocket squares may take some time to grow confident in. The easiest (and by no means wrong) way is to go the white linen TV fold route.
But if you are trying to add some colour to your wardrobe and want to go with silk squares, you can start off by buying a few squares in colours that match your wardrobe.
Paisleys or dots are generally pretty easy to work with as long as you aren't wearing a similar tie. If you're just starting out and don't feel comfortable matching PS, suit, shirt and tie yet, you might want to consider starting to wear your pocket squares in more casual settings with just a sports jacket and shirt and no tie to worry about. Your choice.
As for sizes, many on SF feel that the smaller sized squares keep their shape less well and easily slide down and get lost in their pockets. I guess this depends on your jackets.
For me personally it's the exact opposite, most of my jackets have fairly small chest pockets and work better with squares up to about 32cm or 12" like the ones Kent Wang makes or most pocket squares that are commonly sold in stores here in my country.
Perhaps the small pockets are because many of my jackets are italian, I don't know. But the larger Drake's, Ralph Lauren, Rubinacci squares that measure around 42cm are too large for my pockets.
So consider that.
Hand rolled edges are nice to have and a sign of quality/ workmanship. It's only worth bothering with, if you feel that the rest of your wardrobe is on par with it.
No point in chasing hand rolled pocket squares if you're wearing cheap and worn out shoes IMHO.
White linen square in a TV fold being an exception because the edges will be exposed. So the edges should look nice… or it'll look like a napkin (again, my opinion)
A pocket squares function was also discussed in this thread at some point. In my case it's purely ornamental. I carry a neatly folded cotton hankie and perhaps one or two more paper kleenex tissues in my inner right chest pocket just in case. I wouldn't dream of handing someone my pocket square to sneeze in or doing so myself
I generally wear silk pocket squares in a simple puff and linen ones in a TV fold. Very rarely do I expose the edges or tips on silk squares. It generally seems a little 'too much' for my taste, but can work sometimes… maybe a puff and one corner.
I don't see much point in finding a million fancy ways to fold a square and try to keep it simple. Hold it, puff it, stick it in there.
The 'when or whether to wear a pocket square' question is probably something that you will have to decide on your own.
In my case wearing a square doesn't really affect me in any negative way. I have no OCB guidelines… but for a job interview perhaps I'd tone it down a notch and keep my outfit simpler.
If you know your work environment won't have any issues with you suddenly wearing a PS other than a few surprised remarks, then go for it.