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 $248.59   11.665%   $277.588   $277.59   $277.60   $277.60  
  Purchase #2 $63.50   7.658%   $68.3628   $68.36   $68.35   $68.40  
  Purchase #3 $54.49   11.758%   $60.8969   $60.90   $60.90   $60.90  
  Purchase #4 $250.37   12.773%   $282.3498   $282.35   $282.35   $282.30  
  Purchase #5 $22.95   5.587%   $24.2322   $24.23   $24.25   $24.20  
  Purchase #6 $17.53   4.661%   $18.3471   $18.35   $18.35   $18.30  
  Purchase #7 $59.32   10.274%   $65.4145   $65.41   $65.40   $65.40  
  7 purchase totals = $716.75       $797.1913   $797.19   $797.20   $797.10