Sasha only goes topless for her own site/c4s, and I'm not positive, but I'm pretty sure none of her topless vids are tickling ones. You could check it out, though....
I noticed this too. She's in videos doing most vanilla sexual acts no problem. However she never ever goes topless in any tickling video. Only in one video where she tickle fights on a trampoline do her breasts pop out occasionally. Otherwise, she refuses to go topless, but has tickled countless of topless and naked women, even stimulated a few. It's her choice, but I found this curious.
I noticed this because she's quite a high rated adult model in the vanilla world. She just happens to be also active in the tickling niche.
EDIT: Actually. In her vanilla videos she's actually often clothed as well. It's just her style, I think. Even adult models can be a little reserved.