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 $19.46   4.733%   $20.381   $20.38   $20.40   $20.40  
  Purchase #2 $237.29   9.511%   $259.8587   $259.86   $259.85   $259.90  
  Purchase #3 $191.28   4.379%   $199.6562   $199.66   $199.65   $199.70  
  Purchase #4 $244.65   9.184%   $267.1187   $267.12   $267.10   $267.10  
  Purchase #5 $245.72   10.378%   $271.2208   $271.22   $271.20   $271.20  
  Purchase #6 $87.71   12.767%   $98.9079   $98.91   $98.90   $98.90  
  Purchase #7 $18.32   11.165%   $20.3654   $20.37   $20.35   $20.40  
  7 purchase totals = $1,044.43       $1,137.5087   $1,137.52   $1,137.45   $1,137.60