The problem with letting your ISP decide what spam is, is that you don't know how they set the filters, if you did so would the spammers and they would work round it, hence you don't know what you have not been allowed to have.
Not a problem until a contact you want to here from inadvertantly posts with a subject line your ISP filters, or emails from an ISP with a spam problem so gets onto a black hole list.
The real answer is to have more than one email account. Use a web based free account for anything likley to generate spam for example news group posts, competition entries etc. And keep your real email addy for genuine contacts / business.
Or use a human readable only email address to confuse mail harvesting programmes
e.g.
your dot name at yourisp dot com
instead of
[email protected]