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 $53.95   6.571%   $57.4951   $57.50   $57.50   $57.50  
  Purchase #2 $104.77   8.735%   $113.9217   $113.92   $113.90   $113.90  
  Purchase #3 $48.76   4.177%   $50.7967   $50.80   $50.80   $50.80  
  Purchase #4 $199.32   9.706%   $218.666   $218.67   $218.65   $218.70  
  Purchase #5 $75.59   12.259%   $84.8566   $84.86   $84.85   $84.90  
  Purchase #6 $102.27   4.622%   $106.9969   $107.00   $107.00   $107.00  
  Purchase #7 $206.40   5.704%   $218.1731   $218.17   $218.15   $218.20  
  7 purchase totals = $791.06       $850.9061   $850.92   $850.85   $851.00