With all due respect here, this is one of those fantastic myths that's up there along with no MOT invalidates your insurance.
Insurers are bound by the Financial Ombudsman Service to offer the market value of the vehicle - to do this, we are obliged to use Glass's Guide.
In the event that we have a more specialised vehicle (ie Porsche or even an M3), then we'd check Autotrader as the guide can be out by miles.
By all means dispute the valuation, but you have to suplpy evidence that the value placed on your vehicle by the insurer is incorrect - bear in mind that we use independent engineers a lot, and the FOS will accept their valuations.
The idea is to provide you with enough funds (after the excess has been deducted) to buy an identical vehicle - we all know you can easily find adverts showing the same model going for stupid amounts, but if your insurer can find adverts to support their figures, you're not going to gain anything.
Now this isn't to say you shouldn't dispute the valuation, but rather to do some research first - like I say, if cars are available for the amount they've offered, why the hell should they pay you more?