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 $232.84   11.787%   $260.2849   $260.28   $260.30   $260.30  
  Purchase #2 $49.76   5.607%   $52.55   $52.55   $52.55   $52.60  
  Purchase #3 $157.57   3.706%   $163.4095   $163.41   $163.40   $163.40  
  Purchase #4 $241.32   9.282%   $263.7193   $263.72   $263.70   $263.70  
  Purchase #5 $192.75   10.466%   $212.9232   $212.92   $212.90   $212.90  
  Purchase #6 $17.25   9.177%   $18.833   $18.83   $18.85   $18.80  
  Purchase #7 $60.69   11.335%   $67.5692   $67.57   $67.55   $67.60  
  7 purchase totals = $952.18       $1,039.2891   $1,039.28   $1,039.25   $1,039.30