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 $222.98   11.677%   $249.0174   $249.02   $249.00   $249.00  
  Purchase #2 $43.70   6.381%   $46.4885   $46.49   $46.50   $46.50  
  Purchase #3 $217.86   7.311%   $233.7877   $233.79   $233.80   $233.80  
  Purchase #4 $165.95   7.103%   $177.7374   $177.74   $177.75   $177.70  
  Purchase #5 $226.84   13.393%   $257.2207   $257.22   $257.20   $257.20  
  Purchase #6 $70.39   12.214%   $78.9874   $78.99   $79.00   $79.00  
  Purchase #7 $59.89   11.760%   $66.9331   $66.93   $66.95   $66.90  
  7 purchase totals = $1,007.61       $1,110.1722   $1,110.18   $1,110.20   $1,110.10