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 $79.85   12.311%   $89.6803   $89.68   $89.70   $89.70  
  Purchase #2 $112.99   6.157%   $119.9468   $119.95   $119.95   $119.90  
  Purchase #3 $146.68   4.247%   $152.9095   $152.91   $152.90   $152.90  
  Purchase #4 $241.39   3.378%   $249.5442   $249.54   $249.55   $249.50  
  Purchase #5 $96.76   5.338%   $101.925   $101.93   $101.95   $101.90  
  Purchase #6 $192.51   7.767%   $207.4623   $207.46   $207.45   $207.50  
  Purchase #7 $121.49   11.150%   $135.0361   $135.04   $135.05   $135.00  
  7 purchase totals = $991.67       $1,056.5042   $1,056.51   $1,056.55   $1,056.40