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 $131.81   5.451%   $138.995   $139.00   $139.00   $139.00  
  Purchase #2 $140.73   7.486%   $151.265   $151.27   $151.25   $151.30  
  Purchase #3 $78.70   8.222%   $85.1707   $85.17   $85.15   $85.20  
  Purchase #4 $49.47   3.154%   $51.0303   $51.03   $51.05   $51.00  
  Purchase #5 $79.28   6.518%   $84.4475   $84.45   $84.45   $84.40  
  Purchase #6 $101.53   5.303%   $106.9141   $106.91   $106.90   $106.90  
  Purchase #7 $5.94   5.401%   $6.2608   $6.26   $6.25   $6.30  
  7 purchase totals = $587.46       $624.0834   $624.09   $624.05   $624.10