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 $19.55   4.207%   $20.3725   $20.37   $20.35   $20.40  
  Purchase #2 $122.62   4.309%   $127.9037   $127.90   $127.90   $127.90  
  Purchase #3 $89.35   7.465%   $96.02   $96.02   $96.00   $96.00  
  Purchase #4 $97.81   8.800%   $106.4173   $106.42   $106.40   $106.40  
  Purchase #5 $73.99   7.404%   $79.4682   $79.47   $79.45   $79.50  
  Purchase #6 $27.85   7.262%   $29.8725   $29.87   $29.85   $29.90  
  Purchase #7 $63.71   5.510%   $67.2204   $67.22   $67.20   $67.20  
  7 purchase totals = $494.88       $527.2746   $527.27   $527.15   $527.30