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 $238.69   13.176%   $270.1398   $270.14   $270.15   $270.10  
  Purchase #2 $95.40   9.795%   $104.7444   $104.74   $104.75   $104.70  
  Purchase #3 $24.82   5.224%   $26.1166   $26.12   $26.10   $26.10  
  Purchase #4 $161.30   13.292%   $182.74   $182.74   $182.75   $182.70  
  Purchase #5 $117.39   12.228%   $131.7444   $131.74   $131.75   $131.70  
  Purchase #6 $125.65   13.633%   $142.7799   $142.78   $142.80   $142.80  
  Purchase #7 $27.81   13.807%   $31.6497   $31.65   $31.65   $31.60  
  7 purchase totals = $791.06       $889.9148   $889.91   $889.95   $889.70