I think the answer depends on your financial position, and how worn it actually is.
When it comes to 'stretching things out a bit' I've done just about everything in the book, but I have never worn a tyre to the casing/belting. I run my rear tyres until they have a slick 10mm band forming a flat strip in the middle of the tyre - not saying it's advisable, but I haven't had a problem with doing this. I'm slightly more fussy with the front as it has a detrimental effect on the steering.
As I have said to Graham previously, I tend to switch my front and rear tyres over every year to extend the life of them (the front tyre gets extremely little wear).
Without seeing it it's impossible to say, but if you have any doubt about it I would change it