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 $212.99   12.522%   $239.6606   $239.66   $239.65   $239.70  
  Purchase #2 $234.35   6.866%   $250.4405   $250.44   $250.45   $250.40  
  Purchase #3 $164.62   12.817%   $185.7193   $185.72   $185.70   $185.70  
  Purchase #4 $50.90   10.351%   $56.1687   $56.17   $56.15   $56.20  
  Purchase #5 $183.97   8.700%   $199.9754   $199.98   $200.00   $200.00  
  Purchase #6 $77.39   12.205%   $86.8354   $86.84   $86.85   $86.80  
  Purchase #7 $11.39   11.347%   $12.6824   $12.68   $12.70   $12.70  
  7 purchase totals = $935.61       $1,031.4823   $1,031.49   $1,031.50   $1,031.50