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 $36.85   8.778%   $40.0847   $40.08   $40.10   $40.10  
  Purchase #2 $10.46   12.476%   $11.765   $11.77   $11.75   $11.80  
  Purchase #3 $186.32   12.856%   $210.2733   $210.27   $210.25   $210.30  
  Purchase #4 $168.67   13.223%   $190.9732   $190.97   $190.95   $191.00  
  Purchase #5 $140.44   12.614%   $158.1551   $158.16   $158.15   $158.20  
  Purchase #6 $28.80   9.522%   $31.5423   $31.54   $31.55   $31.50  
  Purchase #7 $179.76   3.635%   $186.2943   $186.29   $186.30   $186.30  
  7 purchase totals = $751.30       $829.0879   $829.08   $829.05   $829.20