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 $208.95   12.316%   $234.6843   $234.68   $234.70   $234.70  
  Purchase #2 $86.37   6.586%   $92.0583   $92.06   $92.05   $92.10  
  Purchase #3 $27.40   9.446%   $29.9882   $29.99   $30.00   $30.00  
  Purchase #4 $238.59   4.619%   $249.6105   $249.61   $249.60   $249.60  
  Purchase #5 $93.69   11.633%   $104.589   $104.59   $104.60   $104.60  
  Purchase #6 $163.71   7.717%   $176.3435   $176.34   $176.35   $176.30  
  Purchase #7 $37.57   13.244%   $42.5458   $42.55   $42.55   $42.50  
  7 purchase totals = $856.28       $929.8196   $929.82   $929.85   $929.80