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 $12.65   9.307%   $13.8273   $13.83   $13.85   $13.80  
  Purchase #2 $70.81   12.351%   $79.5557   $79.56   $79.55   $79.60  
  Purchase #3 $235.31   10.396%   $259.7728   $259.77   $259.75   $259.80  
  Purchase #4 $18.36   5.340%   $19.3404   $19.34   $19.35   $19.30  
  Purchase #5 $233.71   10.301%   $257.7845   $257.78   $257.80   $257.80  
  Purchase #6 $223.73   9.280%   $244.4921   $244.49   $244.50   $244.50  
  Purchase #7 $40.37   12.672%   $45.4857   $45.49   $45.50   $45.50  
  7 purchase totals = $834.94       $920.2585   $920.26   $920.30   $920.30