Summoning

If you’re aiming for someone specific in the banner, then it won’t matter. They always specify that those characters are available even if you haven’t reached that part in the story. So if there is someone who has a rate up in a banner that you want you’ll be fine...as long as you’re lucky lol.

But the other characters who have been added to the story banner over time as chapters and content has released may not be available if you haven’t reached certain chapters. So more of the pool opens up as you progress, so you’d at least have more options outside of the rate up servants on a particular banner. You’ll at least cut down on the chance of doubling up as often.

Also, if you tap the summon info button in the banner screen, you can scroll down and see all the special servants who won’t be available in this banner no matter what. Just a nice bit of info to know if you’re new and are hoping for someone who won’t be showing up again for a long while.

When Stheno is on rateup

Just kidding hehe. What he said, summon a) when you're feeling the gacha itch b) if someone you want is on the banner

A good way to find what you want for non waifu purposes by the way is using different support servants and seeing how they feel. Particularly story and event blue supports.

On one hand, the premium banner gives you an opportunity to get great servants that help out a lot- especially right now with the Camelot 2 banner that gives you a rate-up on Knights of the Round Table
But if you decided to wait, they’ll still be in the Story Summons pool.

It would depend on whether you really wanted a specific servant or if there was a rate up on limited servants.
Servants like Gilgamesh are ideal and they can only be summoned a few times a year at best.

It really does depend on what you want. If you really want a story-locked servant, no sense in rolling before you've unlocked them. I'd say hold off on rolling for now, at least until you've become more familiar with the servant selection and have an idea of which ones you like.