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 $17.50   7.104%   $18.7432   $18.74   $18.75   $18.70  
  Purchase #2 $50.72   11.358%   $56.4808   $56.48   $56.50   $56.50  
  Purchase #3 $82.48   5.280%   $86.8349   $86.83   $86.85   $86.80  
  Purchase #4 $28.69   13.205%   $32.4785   $32.48   $32.50   $32.50  
  Purchase #5 $237.59   10.652%   $262.8981   $262.90   $262.90   $262.90  
  Purchase #6 $15.39   8.378%   $16.6794   $16.68   $16.70   $16.70  
  Purchase #7 $233.37   9.201%   $254.8424   $254.84   $254.85   $254.80  
  7 purchase totals = $665.74       $728.9573   $728.95   $729.05   $728.90