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 $18.65   13.512%   $21.17   $21.17   $21.15   $21.20  
  Purchase #2 $188.84   4.740%   $197.791   $197.79   $197.80   $197.80  
  Purchase #3 $121.57   5.422%   $128.1615   $128.16   $128.15   $128.20  
  Purchase #4 $126.71   6.616%   $135.0931   $135.09   $135.10   $135.10  
  Purchase #5 $131.84   11.430%   $146.9093   $146.91   $146.90   $146.90  
  Purchase #6 $150.66   12.110%   $168.9049   $168.90   $168.90   $168.90  
  Purchase #7 $167.86   7.298%   $180.1104   $180.11   $180.10   $180.10  
  7 purchase totals = $906.13       $978.1402   $978.13   $978.10   $978.20