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 $238.93   12.274%   $268.2563   $268.26   $268.25   $268.30  
  Purchase #2 $68.42   6.285%   $72.7202   $72.72   $72.70   $72.70  
  Purchase #3 $154.63   9.689%   $169.6121   $169.61   $169.60   $169.60  
  Purchase #4 $186.57   11.791%   $208.5685   $208.57   $208.55   $208.60  
  Purchase #5 $167.70   9.121%   $182.9959   $183.00   $183.00   $183.00  
  Purchase #6 $154.70   12.800%   $174.5016   $174.50   $174.50   $174.50  
  Purchase #7 $60.39   11.143%   $67.1193   $67.12   $67.10   $67.10  
  7 purchase totals = $1,031.34       $1,143.7739   $1,143.78   $1,143.70   $1,143.80