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 $214.81   11.261%   $238.9998   $239.00   $239.00   $239.00  
  Purchase #2 $76.60   8.653%   $83.2282   $83.23   $83.25   $83.20  
  Purchase #3 $82.72   6.483%   $88.0827   $88.08   $88.10   $88.10  
  Purchase #4 $113.71   6.833%   $121.4798   $121.48   $121.50   $121.50  
  Purchase #5 $50.48   7.567%   $54.2998   $54.30   $54.30   $54.30  
  Purchase #6 $122.80   9.645%   $134.6441   $134.64   $134.65   $134.60  
  Purchase #7 $195.61   13.283%   $221.5929   $221.59   $221.60   $221.60  
  7 purchase totals = $856.73       $942.3273   $942.32   $942.40   $942.30