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 $21.38   11.749%   $23.8919   $23.89   $23.90   $23.90  
  Purchase #2 $46.34   13.231%   $52.4712   $52.47   $52.45   $52.50  
  Purchase #3 $100.63   7.430%   $108.1068   $108.11   $108.10   $108.10  
  Purchase #4 $182.67   3.355%   $188.7986   $188.80   $188.80   $188.80  
  Purchase #5 $18.97   11.321%   $21.1176   $21.12   $21.10   $21.10  
  Purchase #6 $152.95   11.681%   $170.8161   $170.82   $170.80   $170.80  
  Purchase #7 $77.38   7.379%   $83.0899   $83.09   $83.10   $83.10  
  7 purchase totals = $600.32       $648.2921   $648.30   $648.25   $648.30