If you have other partners who you are bisexual WITH, then yes, absolutely.
Otherwise, what you're talking about is identity, so really, it's about how big a factor it is in who you are.
For instance, while I usually do tell partners of mine my nationality in passing, I don't feel obliged to, because it's a small part of my identity and not only are they likely to find that out as they get to know me, it's not something likely to impinge upon their wants and needs or feelings.
On the other hand, I *am* likely to say from the onset that I'm bisexual, because not only is that a pretty major part of my identity, I'm active with both genders and it IS likely to be something they may want to consider. And I want to be sure I'm with someone who can accept if not embrace that in me. I don't want someone dating me and not knowing the bigger 90and non-negotiable) parts of who I am.
But it's absolutely your call, and only you can figure that out.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson