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 $246.51   11.857%   $275.7387   $275.74   $275.75   $275.70  
  Purchase #2 $54.45   7.531%   $58.5506   $58.55   $58.55   $58.60  
  Purchase #3 $14.35   6.831%   $15.3302   $15.33   $15.35   $15.30  
  Purchase #4 $20.83   12.671%   $23.4694   $23.47   $23.45   $23.50  
  Purchase #5 $125.65   11.127%   $139.6311   $139.63   $139.65   $139.60  
  Purchase #6 $66.67   4.483%   $69.6588   $69.66   $69.65   $69.70  
  Purchase #7 $79.57   3.685%   $82.5022   $82.50   $82.50   $82.50  
  7 purchase totals = $608.03       $664.8810   $664.88   $664.90   $664.90