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 $233.43   11.379%   $259.992   $259.99   $260.00   $260.00  
  Purchase #2 $83.88   13.413%   $95.1308   $95.13   $95.15   $95.10  
  Purchase #3 $213.62   9.533%   $233.9844   $233.98   $234.00   $234.00  
  Purchase #4 $163.35   11.819%   $182.6563   $182.66   $182.65   $182.70  
  Purchase #5 $224.79   11.212%   $249.9935   $249.99   $250.00   $250.00  
  Purchase #6 $41.99   6.308%   $44.6387   $44.64   $44.65   $44.60  
  Purchase #7 $29.66   12.588%   $33.3936   $33.39   $33.40   $33.40  
  7 purchase totals = $990.72       $1,099.7893   $1,099.78   $1,099.85   $1,099.80