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 $206.43   12.255%   $231.728   $231.73   $231.75   $231.70  
  Purchase #2 $21.63   10.353%   $23.8694   $23.87   $23.85   $23.90  
  Purchase #3 $67.47   3.184%   $69.6182   $69.62   $69.60   $69.60  
  Purchase #4 $52.82   10.224%   $58.2203   $58.22   $58.20   $58.20  
  Purchase #5 $167.98   4.649%   $175.7894   $175.79   $175.80   $175.80  
  Purchase #6 $77.98   3.466%   $80.6828   $80.68   $80.70   $80.70  
  Purchase #7 $139.52   8.300%   $151.1002   $151.10   $151.10   $151.10  
  7 purchase totals = $733.83       $791.0083   $791.01   $791.00   $791.00