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 $9.99   6.809%   $10.6702   $10.67   $10.65   $10.70  
  Purchase #2 $175.62   9.759%   $192.7588   $192.76   $192.75   $192.80  
  Purchase #3 $77.87   5.306%   $82.0018   $82.00   $82.00   $82.00  
  Purchase #4 $19.26   11.420%   $21.4595   $21.46   $21.45   $21.50  
  Purchase #5 $91.65   12.124%   $102.7616   $102.76   $102.75   $102.80  
  Purchase #6 $25.75   8.396%   $27.912   $27.91   $27.90   $27.90  
  Purchase #7 $204.30   11.422%   $227.6351   $227.64   $227.65   $227.60  
  7 purchase totals = $604.44       $665.1990   $665.20   $665.15   $665.30