Honestly, I don't think it matters. Unless it is absorbed in the mouth. (and even then, it would be exposed to the mucas membranes longer mixed with food) When I had my wild mustangs I had to squirt the paste wormer into their food. There was NO WAY they were going to let me put it into their mouths, and I never had any problems. They actually get more if mixed into the food, assuming they eat all of the food because they won't be spitting it out. (like mine try to do.) Actually, now that I think about it, generally I shove an alfalfa cube into thier mouth right after I squirt the paste wormer in, to keep them from spitting it out.
I don't know if it really matters either way just as long as it makes it into the mouth. Chanes are, he won't eat it if its in the feed. Horses arent stupid, most can smell bute when its in their feed and it has a scent that is a lot stronger than bute.
I think if your horse takes it even with a fuss, it would be better to do that way. That way you can monitor and make sure he takes in all of it. I think with the feed, it gets dropped all over the container/bucket and you look a lot of the paste he should be getting.
If he is a trouble horse with dewormers and you already know it will be difficult to give it to him, then switching over to injectable dewormers might be something you want to consider. Save the hassle.
I normally mix it in with some wheat bran and stir it pretty good. Most of the time, they just dive into it. It didn't work once because I used a gel instead of a paste, and I couldn't tell if I had mixed it thoroughly enough. Apparently, I didn't. So, I was just wondering why the dewormers aren't mixed into their food, or if it was just because the horse wouldn't eat it.
Okay, next question, can you accidentally make your horse choke by administering it directly into the mouth. I had one syringe that was stiff at first and then it all came exploding out at once. I'm scared it would launch out and either really scare my horse that I'd loose my grip on his mouth or he'd choke. Is it possible?