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 $229.44   12.126%   $257.2619   $257.26   $257.25   $257.30  
  Purchase #2 $89.58   6.723%   $95.6025   $95.60   $95.60   $95.60  
  Purchase #3 $152.28   8.387%   $165.0517   $165.05   $165.05   $165.10  
  Purchase #4 $191.88   12.207%   $215.3028   $215.30   $215.30   $215.30  
  Purchase #5 $73.78   3.825%   $76.6021   $76.60   $76.60   $76.60  
  Purchase #6 $53.25   5.442%   $56.1479   $56.15   $56.15   $56.10  
  Purchase #7 $32.38   5.697%   $34.2247   $34.22   $34.20   $34.20  
  7 purchase totals = $822.59       $900.1936   $900.18   $900.15   $900.20