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 $247.87   12.765%   $279.5106   $279.51   $279.50   $279.50  
  Purchase #2 $114.61   10.230%   $126.3346   $126.33   $126.35   $126.30  
  Purchase #3 $244.26   3.838%   $253.6347   $253.63   $253.65   $253.60  
  Purchase #4 $165.95   4.406%   $173.2618   $173.26   $173.25   $173.30  
  Purchase #5 $242.80   5.521%   $256.205   $256.21   $256.20   $256.20  
  Purchase #6 $41.61   4.650%   $43.5449   $43.54   $43.55   $43.50  
  Purchase #7 $224.89   3.386%   $232.5048   $232.50   $232.50   $232.50  
  7 purchase totals = $1,281.99       $1,364.9964   $1,364.98   $1,365.00   $1,364.90