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 $20.41   5.522%   $21.537   $21.54   $21.55   $21.50  
  Purchase #2 $80.66   10.147%   $88.8446   $88.84   $88.85   $88.80  
  Purchase #3 $77.57   3.848%   $80.5549   $80.55   $80.55   $80.60  
  Purchase #4 $5.73   8.376%   $6.2099   $6.21   $6.20   $6.20  
  Purchase #5 $20.26   4.340%   $21.1393   $21.14   $21.15   $21.10  
  Purchase #6 $156.61   10.694%   $173.3579   $173.36   $173.35   $173.40  
  Purchase #7 $138.74   11.254%   $154.3538   $154.35   $154.35   $154.40  
  7 purchase totals = $499.98       $545.9974   $545.99   $546.00   $546.00