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 $239.93   9.791%   $263.4215   $263.42   $263.40   $263.40  
  Purchase #2 $50.40   13.481%   $57.1944   $57.19   $57.20   $57.20  
  Purchase #3 $155.43   12.547%   $174.9318   $174.93   $174.95   $174.90  
  Purchase #4 $150.84   4.192%   $157.1632   $157.16   $157.15   $157.20  
  Purchase #5 $122.45   6.265%   $130.1215   $130.12   $130.10   $130.10  
  Purchase #6 $116.36   6.659%   $124.1084   $124.11   $124.10   $124.10  
  Purchase #7 $74.49   11.643%   $83.1629   $83.16   $83.15   $83.20  
  7 purchase totals = $909.90       $990.1037   $990.09   $990.05   $990.10