Where is this being practiced? I've been to pretty much every country in Europe in the last 20 years and in a conventional shop the tax (VAT) is always included in the price.
Their conclusion was that if governments wanted to limit consumption, they should choose a policy such that the cost was included in the sticker price, but if they wanted to maximize tax revenue, then they should collect the tax at the register.
The way I understand how VAT taxes work, it would seem that this would be friendlier to consumers, to the detriment, perhaps, of overall consumer spending.
So if you have a wholesale/retail business, you have an easier time of it - everything gets taxed at the same level, and your clients that onsell can claim their own tax credits. It is the final purchaser of the product that ends up actually forking out for the tax.
The advertised price has to be what the consumer will pay. I'm pretty sure some America guy got a Nobel prize in economics for discovering that.
Gasoline (petrol) is always priced with all taxes included.
If the establishment is small enough that they're not using a register for cash transactions, they won't add tax. For instance if you buy a hot dog from a food cart for $4, that will be the whole price.
But for just about anything bought inside an actual store, tax will be added on.
Further, some governments charge tax on the pre-coupon/pre-discounted price, while others tax only the actual final price.
Further still, some entities like public schools are exempt from paying sales taxes in many states/municipalities. In this case, the price sign would have to show both a pre-tax and tax-included price on it, which may be the best solution anyway.
Regardless, these differences are probably easiest to account for at the point of sale.