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 $152.99   12.146%   $171.5722   $171.57   $171.55   $171.60  
  Purchase #2 $58.99   10.759%   $65.3367   $65.34   $65.35   $65.30  
  Purchase #3 $141.72   9.395%   $155.0346   $155.03   $155.05   $155.00  
  Purchase #4 $52.38   9.737%   $57.4802   $57.48   $57.50   $57.50  
  Purchase #5 $180.63   7.397%   $193.9912   $193.99   $194.00   $194.00  
  Purchase #6 $233.50   3.856%   $242.5038   $242.50   $242.50   $242.50  
  Purchase #7 $225.75   7.199%   $242.0017   $242.00   $242.00   $242.00  
  7 purchase totals = $1,045.96       $1,127.9204   $1,127.91   $1,127.95   $1,127.90