I prefix all of my views with 'vw' and make the same requirement of the developers where I work. The main reason you would want to add a prefix is to avoid problems where developers try to give objects the same name. For example, if you have a table named 'promo_ref', you can't have a view named 'promo_ref' (unless you give it a different owner).

I'd hardly call adding 'v' or 'vw' to the name of views "muddling your data model". That's like saying that a penny is muddling Bill Gates' bank account.

What I don't do is use underscores in object names. It's not a rule, just a preference. Underscores slow me down when I'm typing.