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 $45.69   12.463%   $51.3843   $51.38   $51.40   $51.40  
  Purchase #2 $56.67   10.648%   $62.7042   $62.70   $62.70   $62.70  
  Purchase #3 $224.83   9.542%   $246.2833   $246.28   $246.30   $246.30  
  Purchase #4 $134.58   12.767%   $151.7618   $151.76   $151.75   $151.80  
  Purchase #5 $44.41   10.682%   $49.1539   $49.15   $49.15   $49.20  
  Purchase #6 $199.78   7.648%   $215.0592   $215.06   $215.05   $215.10  
  Purchase #7 $79.62   6.749%   $84.9936   $84.99   $85.00   $85.00  
  7 purchase totals = $785.58       $861.3403   $861.32   $861.35   $861.50