i haven't yet sold turnips in another town, but once i do, i'm going to make sure to tip at least 10%.
if someone is giving me a villager for free, i feel the need to leave a thank-you gift behind, whether they ask for it or not. bells, perfect fruit... anything nice.
I tip 10% when I sell turnips, with the occasional perfect fruit basket.
I tipped my friend 1 million for letting me sell turnips. She is a TTer and my turnip price was high so she let me come back and forth I made 8 million but I added a little extra. So in profit I made around 6.5 million bells.
I've never sold turnips in a different town, and I haven't had many chances to tip. The only time I remember tipping was a Jacob's Ladder for one person (though I forget why that was.)
I hosted for turnips a couple of times through reddit, and about half the visitors tipped, someone stole some hybrids (wanker) and one person donated a couple of fossils to my museum. It was quite interesting. I always tip the last amount my turnips sold for (like the last pocketful which is normally around 500k) but I usually try not to bring too many with me as it's a colossal time sink.
I've never tried turnips. If I'm selling perfect fruit then I do it in my second town. I mostly use my second town for signatures and katie now too...but if I did go to someone's town to do either I usually brought perfect fruit as a thank you.