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 $242.74   12.423%   $272.8956   $272.90   $272.90   $272.90  
  Purchase #2 $84.39   12.556%   $94.986   $94.99   $95.00   $95.00  
  Purchase #3 $53.59   4.740%   $56.1302   $56.13   $56.15   $56.10  
  Purchase #4 $106.67   4.771%   $111.7592   $111.76   $111.75   $111.80  
  Purchase #5 $134.25   7.382%   $144.1603   $144.16   $144.15   $144.20  
  Purchase #6 $106.61   8.717%   $115.9032   $115.90   $115.90   $115.90  
  Purchase #7 $42.29   4.302%   $44.1093   $44.11   $44.10   $44.10  
  7 purchase totals = $770.54       $839.9438   $839.95   $839.95   $840.00