Generally it's not a good idea to write a treat for someone if they requested A&D and you want to write a similar prompt but with B/C. But if, let's say, they requested A, B, and C, and left a B/C prompt, writing them a B/C treat without A is probably okay.

The thing a lot of people do when they write treats is they wait to upload the treat until their recip has an actual gift fic uploaded, because otherwise if their assigned writer defaults, the recip may only get the treat and not go out to pinch hits, and many people people want the best for their recip, or they're anxious that the treat might not be all that good, or the treat writer played a little looser with the recip's prompts than they would have with a main gift.