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 $16.74   9.795%   $18.3797   $18.38   $18.40   $18.40  
  Purchase #2 $135.33   9.456%   $148.1268   $148.13   $148.15   $148.10  
  Purchase #3 $23.41   3.627%   $24.2591   $24.26   $24.25   $24.30  
  Purchase #4 $21.85   6.131%   $23.1896   $23.19   $23.20   $23.20  
  Purchase #5 $5.48   7.380%   $5.8844   $5.88   $5.90   $5.90  
  Purchase #6 $100.36   8.779%   $109.1706   $109.17   $109.15   $109.20  
  Purchase #7 $116.90   8.680%   $127.0469   $127.05   $127.05   $127.00  
  7 purchase totals = $420.07       $456.0571   $456.06   $456.10   $456.10