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 $59.53   12.336%   $66.8736   $66.87   $66.85   $66.90  
  Purchase #2 $158.34   12.205%   $177.6654   $177.67   $177.65   $177.70  
  Purchase #3 $122.73   9.646%   $134.5685   $134.57   $134.55   $134.60  
  Purchase #4 $66.32   13.299%   $75.1399   $75.14   $75.15   $75.10  
  Purchase #5 $71.26   6.613%   $75.9724   $75.97   $75.95   $76.00  
  Purchase #6 $125.25   10.628%   $138.5616   $138.56   $138.55   $138.60  
  Purchase #7 $48.82   11.813%   $54.5871   $54.59   $54.60   $54.60  
  7 purchase totals = $652.25       $723.3685   $723.37   $723.30   $723.50