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 $40.90   4.793%   $42.8603   $42.86   $42.85   $42.90  
  Purchase #2 $145.28   9.823%   $159.5509   $159.55   $159.55   $159.60  
  Purchase #3 $20.55   3.362%   $21.2409   $21.24   $21.25   $21.20  
  Purchase #4 $181.97   10.505%   $201.0859   $201.09   $201.10   $201.10  
  Purchase #5 $30.38   8.231%   $32.8806   $32.88   $32.90   $32.90  
  Purchase #6 $12.43   11.627%   $13.8752   $13.88   $13.90   $13.90  
  Purchase #7 $209.49   7.244%   $224.6655   $224.67   $224.65   $224.70  
  7 purchase totals = $641.00       $696.1593   $696.17   $696.20   $696.30