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 $9.52   3.777%   $9.8796   $9.88   $9.90   $9.90  
  Purchase #2 $67.86   9.614%   $74.3841   $74.38   $74.40   $74.40  
  Purchase #3 $211.50   6.867%   $226.0237   $226.02   $226.00   $226.00  
  Purchase #4 $138.79   7.402%   $149.0632   $149.06   $149.05   $149.10  
  Purchase #5 $72.81   7.363%   $78.171   $78.17   $78.15   $78.20  
  Purchase #6 $34.52   6.318%   $36.701   $36.70   $36.70   $36.70  
  Purchase #7 $7.69   5.513%   $8.1139   $8.11   $8.10   $8.10  
  7 purchase totals = $542.69       $582.3365   $582.32   $582.30   $582.40