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 $45.94   8.495%   $49.8426   $49.84   $49.85   $49.80  
  Purchase #2 $74.36   12.181%   $83.4178   $83.42   $83.40   $83.40  
  Purchase #3 $47.55   4.573%   $49.7245   $49.72   $49.70   $49.70  
  Purchase #4 $154.80   5.581%   $163.4394   $163.44   $163.45   $163.40  
  Purchase #5 $155.73   6.281%   $165.5114   $165.51   $165.50   $165.50  
  Purchase #6 $18.66   13.466%   $21.1728   $21.17   $21.15   $21.20  
  Purchase #7 $15.76   7.709%   $16.9749   $16.97   $16.95   $17.00  
  7 purchase totals = $512.80       $550.0834   $550.07   $550.00   $550.00