I don't tend to buy samples."Do or do not. There is no try." Yoda
Sage words from an accomplished Jedi.
I usually avoid samples as well unless it's in a "sample pack" where I can try a few varieties of say, several grades of Baozhong, etc -- then decide where I want to concentrate my money in a future purchase. Those make sense.
25g is a bit more than a sample, IMO. 10g does qualify as sample sized, though with some teas, you may need more than 10g for a proper brewing. Like Mr. Baymore, I just tend to go for 50g (100g if sencha), because then at least I have more than two chances to get it right.
I think there is just no cut and dry answer to this question given the breadth of tea styles and quality offered by online vendors.
Exploring Kame-Jiru Shi this morning. I'm beginning to improve my gyo brewing...I think.