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 $126.56   10.303%   $139.5995   $139.60   $139.60   $139.60  
  Purchase #2 $225.33   8.743%   $245.0306   $245.03   $245.05   $245.00  
  Purchase #3 $227.85   12.717%   $256.8257   $256.83   $256.85   $256.80  
  Purchase #4 $164.32   4.845%   $172.2813   $172.28   $172.30   $172.30  
  Purchase #5 $185.55   8.858%   $201.986   $201.99   $202.00   $202.00  
  Purchase #6 $55.50   8.587%   $60.2658   $60.27   $60.25   $60.30  
  Purchase #7 $188.31   3.164%   $194.2681   $194.27   $194.25   $194.30  
  7 purchase totals = $1,173.42       $1,270.2570   $1,270.27   $1,270.30   $1,270.30