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 $227.62   9.831%   $249.9973   $250.00   $250.00   $250.00  
  Purchase #2 $245.51   13.328%   $278.2316   $278.23   $278.25   $278.20  
  Purchase #3 $59.91   13.535%   $68.0188   $68.02   $68.00   $68.00  
  Purchase #4 $21.35   6.334%   $22.7023   $22.70   $22.70   $22.70  
  Purchase #5 $234.36   8.575%   $254.4564   $254.46   $254.45   $254.50  
  Purchase #6 $189.53   13.425%   $214.9744   $214.97   $214.95   $215.00  
  Purchase #7 $141.66   12.720%   $159.6792   $159.68   $159.70   $159.70  
  7 purchase totals = $1,119.94       $1,248.0600   $1,248.06   $1,248.05   $1,248.10