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 $249.59   12.814%   $281.5725   $281.57   $281.55   $281.60  
  Purchase #2 $17.81   8.236%   $19.2768   $19.28   $19.30   $19.30  
  Purchase #3 $183.27   4.752%   $191.979   $191.98   $192.00   $192.00  
  Purchase #4 $32.48   9.628%   $35.6072   $35.61   $35.60   $35.60  
  Purchase #5 $230.56   10.142%   $253.9434   $253.94   $253.95   $253.90  
  Purchase #6 $224.65   11.135%   $249.6648   $249.66   $249.65   $249.70  
  Purchase #7 $60.73   10.643%   $67.1935   $67.19   $67.20   $67.20  
  7 purchase totals = $999.09       $1,099.2372   $1,099.23   $1,099.25   $1,099.30