Just the opposite is probably more important. Since sales taxes can vary locally (city, county, state) hiding the real price prevents people from noticing they can get cheaper goods by driving across an imaginary line.
Example: There is a walmart in both Cityville and its suburb Township. Cityville has a 9% sales tax but Township has a 7% tax rate. Both stores can advertise that they're selling a tv for $500, even though it costs $545 in Cityville but $535 in Township.
This helps prevent people from driving to the Township store to save money, because the advertised price is the same and most people won't think about the tax rate difference.
Example: There is a walmart in both Cityville and its suburb Township. Cityville has a 9% sales tax but Township has a 7% tax rate. Both stores can advertise that they're selling a tv for $500, even though it costs $545 in Cityville but $535 in Township.
This helps prevent people from driving to the Township store to save money, because the advertised price is the same and most people won't think about the tax rate difference.