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 $249.95   7.474%   $268.6313   $268.63   $268.65   $268.60  
  Purchase #2 $14.37   12.792%   $16.2082   $16.21   $16.20   $16.20  
  Purchase #3 $247.72   8.158%   $267.929   $267.93   $267.95   $267.90  
  Purchase #4 $23.81   7.526%   $25.6019   $25.60   $25.60   $25.60  
  Purchase #5 $33.46   10.670%   $37.0302   $37.03   $37.05   $37.00  
  Purchase #6 $125.34   13.684%   $142.4915   $142.49   $142.50   $142.50  
  Purchase #7 $7.71   8.726%   $8.3828   $8.38   $8.40   $8.40  
  7 purchase totals = $702.36       $766.2749   $766.27   $766.35   $766.20