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 $224.63   3.647%   $232.8223   $232.82   $232.80   $232.80  
  Purchase #2 $14.72   5.388%   $15.5131   $15.51   $15.50   $15.50  
  Purchase #3 $37.46   3.125%   $38.6306   $38.63   $38.65   $38.60  
  Purchase #4 $7.65   7.238%   $8.2037   $8.20   $8.20   $8.20  
  Purchase #5 $124.60   4.427%   $130.116   $130.12   $130.10   $130.10  
  Purchase #6 $126.82   6.370%   $134.8984   $134.90   $134.90   $134.90  
  Purchase #7 $77.26   3.803%   $80.1982   $80.20   $80.20   $80.20  
  7 purchase totals = $613.14       $640.3823   $640.38   $640.35   $640.30