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 $189.99   11.573%   $211.9775   $211.98   $212.00   $212.00  
  Purchase #2 $177.73   10.412%   $196.2352   $196.24   $196.25   $196.20  
  Purchase #3 $72.61   8.186%   $78.5539   $78.55   $78.55   $78.60  
  Purchase #4 $56.25   4.833%   $58.9686   $58.97   $58.95   $59.00  
  Purchase #5 $87.55   6.179%   $92.9597   $92.96   $92.95   $93.00  
  Purchase #6 $240.81   13.451%   $273.2014   $273.20   $273.20   $273.20  
  Purchase #7 $10.95   5.562%   $11.559   $11.56   $11.55   $11.60  
  7 purchase totals = $835.89       $923.4553   $923.46   $923.45   $923.60