Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $25.63   12.724%   $28.8912   $28.89   $28.90   $28.90  
  Purchase #2 $84.59   6.391%   $89.9961   $90.00   $90.00   $90.00  
  Purchase #3 $197.77   3.658%   $205.0044   $205.00   $205.00   $205.00  
  Purchase #4 $51.47   11.214%   $57.2418   $57.24   $57.25   $57.20  
  Purchase #5 $207.36   10.874%   $229.9083   $229.91   $229.90   $229.90  
  Purchase #6 $204.32   12.366%   $229.5862   $229.59   $229.60   $229.60  
  Purchase #7 $210.35   13.602%   $238.9618   $238.96   $238.95   $239.00  
  7 purchase totals = $981.49       $1,079.5898   $1,079.59   $1,079.60   $1,079.60