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 $241.43   12.287%   $271.0945   $271.09   $271.10   $271.10  
  Purchase #2 $245.63   13.288%   $278.2693   $278.27   $278.25   $278.30  
  Purchase #3 $44.90   13.597%   $51.0051   $51.01   $51.00   $51.00  
  Purchase #4 $232.77   9.548%   $254.9949   $254.99   $255.00   $255.00  
  Purchase #5 $119.52   6.708%   $127.5374   $127.54   $127.55   $127.50  
  Purchase #6 $185.36   12.334%   $208.2223   $208.22   $208.20   $208.20  
  Purchase #7 $34.63   13.565%   $39.3276   $39.33   $39.35   $39.30  
  7 purchase totals = $1,104.24       $1,230.4511   $1,230.45   $1,230.45   $1,230.40