I made the edits listed by frank18 to a vanilla 1.5.3 and it didn't fix the admin displayed price issue so further help is going to be needed.
The store side is showing the correct price of 6.99 but admin stubbornly shows 7.00

Making the Net price only 2 decimal places wouldn't fix the problem as 20% tax on 5.82 is 6.984
It will only work if it uses the full 5.825 value.
It must be feasible as the store side can calculate it from the figure stored in the database.