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 $248.56   12.493%   $279.6126   $279.61   $279.60   $279.60  
  Purchase #2 $58.53   8.211%   $63.3359   $63.34   $63.35   $63.30  
  Purchase #3 $109.60   7.433%   $117.7466   $117.75   $117.75   $117.70  
  Purchase #4 $14.62   6.718%   $15.6022   $15.60   $15.60   $15.60  
  Purchase #5 $214.71   13.133%   $242.9079   $242.91   $242.90   $242.90  
  Purchase #6 $139.96   12.765%   $157.8259   $157.83   $157.85   $157.80  
  Purchase #7 $133.45   11.737%   $149.113   $149.11   $149.10   $149.10  
  7 purchase totals = $919.43       $1,026.1441   $1,026.15   $1,026.15   $1,026.00