I think this metric changes once you get past your first few out-bound programs. sort of a moving target based on list source...I Look mostly at compairing programs against each other and tracking the lift in deliverability. also it is key to do all the steps Marketo recomends (CName, SPF & DKIM) as well as brand the traclking links in your emails. Here is the documenttion on that.
It really depends on who you are sending the emails to. If its to your current customers anything over 1% is not ideal, but if they are it could be a case that your customer database is old and some customers may now be using a new email address.
If you click on a bounced email and then go into Lead Info, you will fiund a field called "Email Invalid Cause" this will give you an idea of why they bounced. So might be worthwhile exporting out all your bounced emails and filter through that field and you will see where the majority of bounces happed.
If its bouncing because of spam....you could have yourself a few issues in the future whereby mailservers will blacklist your company email as spam. In this case you might want to try and see if you can spam score your mails before you send them out. This can be done using the IBM deliverabilty tool if you use that or there are some less accurate but still useful websites online that can help you do this and give pointers on where you might be at a high risk of spam!
I aim for a deliverability rate of over 95%. The higher the better as it means your emails are making it into your prospects inboxes. Its a good idea to routinely (quarterly or even monthly) cleanse your email database and remove potential spam traps and non responsive emaisl. I highly recommend looking into companies such as NetProspex or ReachForce as they can cleanse your list and have supplemental tools to keep the your email database clean moving forward.