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 $131.26   4.366%   $136.9908   $136.99   $137.00   $137.00  
  Purchase #2 $127.34   3.169%   $131.3754   $131.38   $131.40   $131.40  
  Purchase #3 $15.53   12.252%   $17.4327   $17.43   $17.45   $17.40  
  Purchase #4 $137.35   4.503%   $143.5349   $143.53   $143.55   $143.50  
  Purchase #5 $5.77   5.261%   $6.0736   $6.07   $6.05   $6.10  
  Purchase #6 $117.51   7.542%   $126.3726   $126.37   $126.35   $126.40  
  Purchase #7 $71.33   9.861%   $78.3639   $78.36   $78.35   $78.40  
  7 purchase totals = $606.09       $640.1439   $640.13   $640.15   $640.20