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 $17.75   11.179%   $19.7343   $19.73   $19.75   $19.70  
  Purchase #2 $135.99   6.199%   $144.42   $144.42   $144.40   $144.40  
  Purchase #3 $160.64   3.715%   $166.6078   $166.61   $166.60   $166.60  
  Purchase #4 $14.46   7.104%   $15.4872   $15.49   $15.50   $15.50  
  Purchase #5 $143.52   8.342%   $155.4924   $155.49   $155.50   $155.50  
  Purchase #6 $137.97   12.343%   $154.9996   $155.00   $155.00   $155.00  
  Purchase #7 $18.97   8.727%   $20.6255   $20.63   $20.65   $20.60  
  7 purchase totals = $629.30       $677.3668   $677.37   $677.40   $677.30