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 $16.66   8.543%   $18.0833   $18.08   $18.10   $18.10  
  Purchase #2 $19.47   7.639%   $20.9573   $20.96   $20.95   $21.00  
  Purchase #3 $42.66   3.754%   $44.2615   $44.26   $44.25   $44.30  
  Purchase #4 $40.32   12.493%   $45.3572   $45.36   $45.35   $45.40  
  Purchase #5 $103.74   12.488%   $116.6951   $116.70   $116.70   $116.70  
  Purchase #6 $154.69   4.141%   $161.0957   $161.10   $161.10   $161.10  
  Purchase #7 $81.73   10.467%   $90.2847   $90.28   $90.30   $90.30  
  7 purchase totals = $459.27       $496.7348   $496.74   $496.75   $496.90