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 $17.74   3.639%   $18.3856   $18.39   $18.40   $18.40  
  Purchase #2 $160.37   6.245%   $170.3851   $170.39   $170.40   $170.40  
  Purchase #3 $78.48   6.668%   $83.713   $83.71   $83.70   $83.70  
  Purchase #4 $51.76   13.866%   $58.937   $58.94   $58.95   $58.90  
  Purchase #5 $17.27   9.413%   $18.8956   $18.90   $18.90   $18.90  
  Purchase #6 $91.72   8.394%   $99.419   $99.42   $99.40   $99.40  
  Purchase #7 $181.70   6.727%   $193.923   $193.92   $193.90   $193.90  
  7 purchase totals = $599.04       $643.6583   $643.67   $643.65   $643.60