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 $9.45   3.543%   $9.7848   $9.78   $9.80   $9.80  
  Purchase #2 $42.51   9.123%   $46.3882   $46.39   $46.40   $46.40  
  Purchase #3 $10.50   7.454%   $11.2827   $11.28   $11.30   $11.30  
  Purchase #4 $90.70   11.125%   $100.7904   $100.79   $100.80   $100.80  
  Purchase #5 $183.99   13.226%   $208.3245   $208.32   $208.30   $208.30  
  Purchase #6 $245.84   4.865%   $257.8001   $257.80   $257.80   $257.80  
  Purchase #7 $31.35   11.238%   $34.8731   $34.87   $34.85   $34.90  
  7 purchase totals = $614.34       $669.2438   $669.23   $669.25   $669.30