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 $213.34   12.478%   $239.9606   $239.96   $239.95   $240.00  
  Purchase #2 $94.54   11.188%   $105.1171   $105.12   $105.10   $105.10  
  Purchase #3 $218.65   9.623%   $239.6907   $239.69   $239.70   $239.70  
  Purchase #4 $88.69   11.132%   $98.563   $98.56   $98.55   $98.60  
  Purchase #5 $6.92   6.127%   $7.344   $7.34   $7.35   $7.30  
  Purchase #6 $248.71   5.741%   $262.9884   $262.99   $263.00   $263.00  
  Purchase #7 $233.93   12.505%   $263.1829   $263.18   $263.20   $263.20  
  7 purchase totals = $1,104.78       $1,216.8467   $1,216.84   $1,216.85   $1,216.90