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 $242.61   8.634%   $263.5569   $263.56   $263.55   $263.60  
  Purchase #2 $65.96   13.152%   $74.6351   $74.64   $74.65   $74.60  
  Purchase #3 $197.90   13.667%   $224.947   $224.95   $224.95   $224.90  
  Purchase #4 $106.45   11.364%   $118.547   $118.55   $118.55   $118.50  
  Purchase #5 $175.99   8.767%   $191.419   $191.42   $191.40   $191.40  
  Purchase #6 $51.36   7.568%   $55.2469   $55.25   $55.25   $55.20  
  Purchase #7 $37.81   11.392%   $42.1173   $42.12   $42.10   $42.10  
  7 purchase totals = $878.08       $970.4692   $970.49   $970.45   $970.30