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 $170.32   4.429%   $177.8635   $177.86   $177.85   $177.90  
  Purchase #2 $170.66   9.234%   $186.4187   $186.42   $186.40   $186.40  
  Purchase #3 $30.57   5.433%   $32.2309   $32.23   $32.25   $32.20  
  Purchase #4 $170.73   8.785%   $185.7286   $185.73   $185.75   $185.70  
  Purchase #5 $167.67   12.345%   $188.3689   $188.37   $188.35   $188.40  
  Purchase #6 $225.65   4.602%   $236.0344   $236.03   $236.05   $236.00  
  Purchase #7 $209.72   11.183%   $233.173   $233.17   $233.15   $233.20  
  7 purchase totals = $1,145.32       $1,239.8180   $1,239.81   $1,239.80   $1,239.80