I said this in a question earlier...today I think. The threshold for bounce rate creating a negative impact for SEO is very high, to the point where you don't need to worry about it.
With regards to using Pinterest as a marketing channel, ask yourself what the benefit to your user is. Are you going to create intriguing SEO images that people will want to save or share? If so, I don't think I'd spend that capital in Pinterest, rather I'd try to turn it into good link bait.
I haven't seen anything good on whether or not Pinterest generates a measurable social signal that could influence a SERP, so I can't speak to that.
