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 $108.83   13.299%   $123.3033   $123.30   $123.30   $123.30  
  Purchase #2 $211.92   10.157%   $233.4447   $233.44   $233.45   $233.40  
  Purchase #3 $224.71   4.745%   $235.3725   $235.37   $235.35   $235.40  
  Purchase #4 $92.37   10.757%   $102.3062   $102.31   $102.30   $102.30  
  Purchase #5 $86.25   12.149%   $96.7285   $96.73   $96.75   $96.70  
  Purchase #6 $30.50   11.332%   $33.9563   $33.96   $33.95   $34.00  
  Purchase #7 $125.64   4.251%   $130.981   $130.98   $131.00   $131.00  
  7 purchase totals = $880.22       $956.0925   $956.09   $956.10   $956.10