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 $9.99   9.394%   $10.9285   $10.93   $10.95   $10.90  
  Purchase #2 $188.62   6.175%   $200.2673   $200.27   $200.25   $200.30  
  Purchase #3 $35.40   13.571%   $40.2041   $40.20   $40.20   $40.20  
  Purchase #4 $103.65   6.564%   $110.4536   $110.45   $110.45   $110.50  
  Purchase #5 $197.59   13.725%   $224.7092   $224.71   $224.70   $224.70  
  Purchase #6 $6.84   5.112%   $7.1897   $7.19   $7.20   $7.20  
  Purchase #7 $70.65   11.500%   $78.7748   $78.77   $78.75   $78.80  
  7 purchase totals = $612.74       $672.5272   $672.52   $672.50   $672.60