I think it depends on what you want to achieve with your slogan.
Brand enhancing slogans
If the main reason is brand familiarity then I would position it as close to the logo/branding element as possible, in a way that support.
For instance: Philips, sense and simplicity.
This will make people go, ow cool Philips makes things that makes sense and are simple to use...awesome. Another good example, I think, is Nike and Just do it.
Product enhancing slogans
These are slogans/taglines that add value to the product/service one is selling.
Philips Led: Brightens up your nights (I'm totally making this up but you get the point). This is directly related to the lightning business of Philips, e.g. product specific.
Nike fuel: Motivation on your wrist. Again a tagline that enhances the product/service.
These are usually found at the center of the page, near the product service that is sold and apart from the main branding of the company.