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 $19.97   7.442%   $21.4562   $21.46   $21.45   $21.50  
  Purchase #2 $58.86   8.859%   $64.0744   $64.07   $64.05   $64.10  
  Purchase #3 $110.81   3.840%   $115.0651   $115.07   $115.05   $115.10  
  Purchase #4 $118.91   3.159%   $122.6664   $122.67   $122.65   $122.70  
  Purchase #5 $154.46   7.643%   $166.2654   $166.27   $166.25   $166.30  
  Purchase #6 $202.68   6.587%   $216.0305   $216.03   $216.05   $216.00  
  Purchase #7 $170.91   7.403%   $183.5625   $183.56   $183.55   $183.60  
  7 purchase totals = $836.60       $889.1205   $889.13   $889.05   $889.30