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 $133.70   3.864%   $138.8662   $138.87   $138.85   $138.90  
  Purchase #2 $209.89   5.604%   $221.6522   $221.65   $221.65   $221.70  
  Purchase #3 $240.77   13.477%   $273.2186   $273.22   $273.20   $273.20  
  Purchase #4 $154.27   4.516%   $161.2368   $161.24   $161.25   $161.20  
  Purchase #5 $172.73   7.137%   $185.0577   $185.06   $185.05   $185.10  
  Purchase #6 $21.42   4.838%   $22.4563   $22.46   $22.45   $22.50  
  Purchase #7 $154.70   6.835%   $165.2737   $165.27   $165.25   $165.30  
  7 purchase totals = $1,087.48       $1,167.7615   $1,167.77   $1,167.70   $1,167.90