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 $240.98   6.854%   $257.4968   $257.50   $257.50   $257.50  
  Purchase #2 $73.59   8.183%   $79.6119   $79.61   $79.60   $79.60  
  Purchase #3 $216.64   10.210%   $238.7589   $238.76   $238.75   $238.80  
  Purchase #4 $77.79   4.247%   $81.0937   $81.09   $81.10   $81.10  
  Purchase #5 $55.94   5.776%   $59.1711   $59.17   $59.15   $59.20  
  Purchase #6 $173.98   8.796%   $189.2833   $189.28   $189.30   $189.30  
  Purchase #7 $234.70   4.637%   $245.583   $245.58   $245.60   $245.60  
  7 purchase totals = $1,073.62       $1,150.9987   $1,150.99   $1,151.00   $1,151.10