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 $161.59   13.444%   $183.3142   $183.31   $183.30   $183.30  
  Purchase #2 $115.47   10.656%   $127.7745   $127.77   $127.75   $127.80  
  Purchase #3 $19.93   3.494%   $20.6264   $20.63   $20.65   $20.60  
  Purchase #4 $125.91   12.179%   $141.2446   $141.24   $141.25   $141.20  
  Purchase #5 $101.70   9.748%   $111.6137   $111.61   $111.60   $111.60  
  Purchase #6 $70.75   3.142%   $72.973   $72.97   $72.95   $73.00  
  Purchase #7 $220.35   8.804%   $239.7496   $239.75   $239.75   $239.70  
  7 purchase totals = $815.70       $897.2960   $897.28   $897.25   $897.20