Here's a couple more threads! One is more about the right day, but people talk about timing choices, too:
http://forums.tapastic.com/t/what-time-do-you-normally-post-your-comic/9671
http://forums.tapastic.com/t/best-days-for-uploading-and-if-you-miss-it/8771
I post at midnight EST because that's when my website updates, and updating at midnight is kind of a webcomics tradition, but when I was testing out different times, I got the best results posting at EST's lunchtime. I think there is a bit of a difference, but you'd have to kinda figure out what's best for you personally (depending on your audience, etc), and updating at a non-optimal time doesn't seem to be a big setback. I don't think Tapastic has the same "dead zones" that social media has.
TL;DR there's little trends you can look for, but as long as you're consistent imo you can make most anything work.