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 $25.38   10.625%   $28.0766   $28.08   $28.10   $28.10  
  Purchase #2 $15.66   3.534%   $16.2134   $16.21   $16.20   $16.20  
  Purchase #3 $98.25   5.544%   $103.697   $103.70   $103.70   $103.70  
  Purchase #4 $127.40   3.710%   $132.1265   $132.13   $132.15   $132.10  
  Purchase #5 $106.68   11.200%   $118.6282   $118.63   $118.65   $118.60  
  Purchase #6 $78.34   5.487%   $82.6385   $82.64   $82.65   $82.60  
  Purchase #7 $143.72   7.401%   $154.3567   $154.36   $154.35   $154.40  
  7 purchase totals = $595.43       $635.7369   $635.75   $635.80   $635.70