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 $6.27   6.438%   $6.6737   $6.67   $6.65   $6.70  
  Purchase #2 $122.80   5.858%   $129.9936   $129.99   $130.00   $130.00  
  Purchase #3 $40.29   10.639%   $44.5765   $44.58   $44.60   $44.60  
  Purchase #4 $69.94   5.251%   $73.6125   $73.61   $73.60   $73.60  
  Purchase #5 $180.62   5.189%   $189.9924   $189.99   $190.00   $190.00  
  Purchase #6 $123.59   9.577%   $135.4262   $135.43   $135.45   $135.40  
  Purchase #7 $137.60   12.190%   $154.3734   $154.37   $154.35   $154.40  
  7 purchase totals = $681.11       $734.6483   $734.64   $734.65   $734.70