What's the best way to use wishes in Genshin Impact?

It doesn't make a difference, besides for speed and flexing your 10-pull.

It's not just a claim, it's literally written in the game that you will get a guaranteed 4 star every 10 pulls and a guaranteed 5 star every 90 pulls (character banner) or 80 pulls (weapon banner).

Here's the relevant text for the current limited banner:

5-Star Items For Event Wish - Sparkling Steps: Base probability of winning 5-star character = 0.600%; consolidated probability (incl. guarantee) = 1.600%; guaranteed to win 5-star character at least once per 90 attempts.

4-Star Items For Event Wish - Sparkling Steps: Base probability of winning 4-star item = 5.100%; base probability of winning 4-star character = 2.550%, and base probability of winning 4-star weapon = 2.550%; consolidated probability (incl. guarantee) of winning 4-star item = 13.000%; guaranteed to win 4-star or above item at least once per 10 attempts.

There is no mention of 10-pulls - everything is counted per attempt and hence 10-pulls do not affect the guarantee nor odds.

There’s no difference in statistics/pity pulls, but if you do 1x pulls then you’re more likely to have some left over after you get the character you want. Say you do a 10x pull on Ganyu’s banner and get her- she could have been the first pull in a line of 10, and now those other 9 wishes are wasted when they could have been used for other banners. Basically just helps to stop while you’re ahead!