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 $250.70   13.610%   $284.8203   $284.82   $284.80   $284.80  
  Purchase #2 $169.53   11.268%   $188.6326   $188.63   $188.65   $188.60  
  Purchase #3 $213.47   6.859%   $228.1119   $228.11   $228.10   $228.10  
  Purchase #4 $240.70   10.285%   $265.456   $265.46   $265.45   $265.50  
  Purchase #5 $240.63   10.811%   $266.6445   $266.64   $266.65   $266.60  
  Purchase #6 $246.74   13.211%   $279.3368   $279.34   $279.35   $279.30  
  Purchase #7 $206.71   4.394%   $215.7928   $215.79   $215.80   $215.80  
  7 purchase totals = $1,568.48       $1,728.7949   $1,728.79   $1,728.80   $1,728.70