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 $18.89   7.788%   $20.3612   $20.36   $20.35   $20.40  
  Purchase #2 $131.72   8.582%   $143.0242   $143.02   $143.00   $143.00  
  Purchase #3 $18.55   9.733%   $20.3555   $20.36   $20.35   $20.40  
  Purchase #4 $122.47   8.525%   $132.9106   $132.91   $132.90   $132.90  
  Purchase #5 $100.43   9.216%   $109.6856   $109.69   $109.70   $109.70  
  Purchase #6 $230.38   9.703%   $252.7338   $252.73   $252.75   $252.70  
  Purchase #7 $41.61   7.547%   $44.7503   $44.75   $44.75   $44.80  
  7 purchase totals = $664.05       $723.8212   $723.82   $723.80   $723.90