I flip the front of mines as they were getting bald. Only time I wouldn't flip it is if it's so worn that you see the belts.

 

I autocross and as such have funky wear sometimes. Flipping definitely helps make them last. I've flipped R-comps even when I've seen cords before.

If it was for a set of wheels and tires that see daily driving, I probably wouldn't flip if it is very heavily worn. Keep an eye on it, if you catch it early, you can prolong the usefulness of the tire.

Having a tire dismounted, mounted, and rebalanced only costs around $15. I don't even bother rebalancing sometimes since the wheel is usually balanced and the tire can spin relative to the rim when autocrossing. It's a lot cheaper than shelling out for new tires when most of the tire is still good.

 

If your front tires are already making the famous roar noise it's too late to flip em! Best advice, buy new tires and get an alignment by a qualified alignment shop....that means no Sears or PepBoys but an independent alignment shop that knows how!
Gary