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 $18.99   7.541%   $20.422   $20.42   $20.40   $20.40  
  Purchase #2 $104.40   4.217%   $108.8025   $108.80   $108.80   $108.80  
  Purchase #3 $90.64   12.268%   $101.7597   $101.76   $101.75   $101.80  
  Purchase #4 $131.97   10.726%   $146.1251   $146.13   $146.15   $146.10  
  Purchase #5 $33.35   3.506%   $34.5193   $34.52   $34.50   $34.50  
  Purchase #6 $219.58   9.497%   $240.4335   $240.43   $240.45   $240.40  
  Purchase #7 $12.29   5.282%   $12.9392   $12.94   $12.95   $12.90  
  7 purchase totals = $611.22       $665.0013   $665.00   $665.00   $664.90