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 $238.97   12.398%   $268.5975   $268.60   $268.60   $268.60  
  Purchase #2 $101.94   4.351%   $106.3754   $106.38   $106.40   $106.40  
  Purchase #3 $25.87   6.430%   $27.5334   $27.53   $27.55   $27.50  
  Purchase #4 $56.84   6.640%   $60.6142   $60.61   $60.60   $60.60  
  Purchase #5 $48.45   11.275%   $53.9127   $53.91   $53.90   $53.90  
  Purchase #6 $133.86   13.423%   $151.828   $151.83   $151.85   $151.80  
  Purchase #7 $5.72   12.530%   $6.4367   $6.44   $6.45   $6.40  
  7 purchase totals = $611.65       $675.2979   $675.30   $675.35   $675.20