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 $24.63   11.162%   $27.3792   $27.38   $27.40   $27.40  
  Purchase #2 $236.87   13.547%   $268.9588   $268.96   $268.95   $269.00  
  Purchase #3 $214.99   5.734%   $227.3175   $227.32   $227.30   $227.30  
  Purchase #4 $56.68   12.198%   $63.5938   $63.59   $63.60   $63.60  
  Purchase #5 $244.60   6.574%   $260.68   $260.68   $260.70   $260.70  
  Purchase #6 $236.97   13.569%   $269.1245   $269.12   $269.10   $269.10  
  Purchase #7 $85.86   4.308%   $89.5588   $89.56   $89.55   $89.60  
  7 purchase totals = $1,100.60       $1,206.6126   $1,206.61   $1,206.60   $1,206.70