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 $233.44   9.684%   $256.0463   $256.05   $256.05   $256.00  
  Purchase #2 $17.78   11.132%   $19.7593   $19.76   $19.75   $19.80  
  Purchase #3 $95.80   11.863%   $107.1648   $107.16   $107.15   $107.20  
  Purchase #4 $117.94   5.587%   $124.5293   $124.53   $124.55   $124.50  
  Purchase #5 $108.94   9.399%   $119.1793   $119.18   $119.20   $119.20  
  Purchase #6 $78.60   12.524%   $88.4439   $88.44   $88.45   $88.40  
  Purchase #7 $231.85   9.278%   $253.361   $253.36   $253.35   $253.40  
  7 purchase totals = $884.35       $968.4839   $968.48   $968.50   $968.50