The trouble is that sales tax can be different in every municipality. National advertising would be a nightmare. However, I think prices at brick and mortar stores should be tax included and, when shopping online, if my address is known, the tax should be include as well.
I also think "plus Tax/Tax included" should be featured more prominently but I think that businesses would likely do that themselves given the conditions above so that, when comparing prices, you would very noticeably see that whether tax was included or not in your price. ie, Amazon would put in green letters near the price "Tax included" so when I compared their price to another place I would know why Amazon's price might be higher.