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 $17.83   12.482%   $20.0555   $20.06   $20.05   $20.10  
  Purchase #2 $13.89   13.571%   $15.775   $15.78   $15.80   $15.80  
  Purchase #3 $91.46   13.417%   $103.7312   $103.73   $103.75   $103.70  
  Purchase #4 $40.78   10.767%   $45.1708   $45.17   $45.15   $45.20  
  Purchase #5 $182.80   8.165%   $197.7256   $197.73   $197.75   $197.70  
  Purchase #6 $145.62   5.251%   $153.2665   $153.27   $153.25   $153.30  
  Purchase #7 $104.66   7.350%   $112.3525   $112.35   $112.35   $112.40  
  7 purchase totals = $597.04       $648.0771   $648.09   $648.10   $648.20