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 $26.78   4.490%   $27.9824   $27.98   $28.00   $28.00  
  Purchase #2 $109.73   8.370%   $118.9144   $118.91   $118.90   $118.90  
  Purchase #3 $27.90   12.397%   $31.3588   $31.36   $31.35   $31.40  
  Purchase #4 $9.68   5.177%   $10.1811   $10.18   $10.20   $10.20  
  Purchase #5 $6.48   3.841%   $6.7289   $6.73   $6.75   $6.70  
  Purchase #6 $141.75   6.440%   $150.8787   $150.88   $150.90   $150.90  
  Purchase #7 $168.27   4.225%   $175.3794   $175.38   $175.40   $175.40  
  7 purchase totals = $490.59       $521.4237   $521.42   $521.50   $521.50