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 $28.82   4.248%   $30.0443   $30.04   $30.05   $30.00  
  Purchase #2 $20.49   11.233%   $22.7916   $22.79   $22.80   $22.80  
  Purchase #3 $248.71   7.834%   $268.1939   $268.19   $268.20   $268.20  
  Purchase #4 $140.87   8.427%   $152.7411   $152.74   $152.75   $152.70  
  Purchase #5 $205.68   8.754%   $223.6852   $223.69   $223.70   $223.70  
  Purchase #6 $122.42   4.372%   $127.7722   $127.77   $127.75   $127.80  
  Purchase #7 $69.97   12.667%   $78.8331   $78.83   $78.85   $78.80  
  7 purchase totals = $836.96       $904.0614   $904.05   $904.10   $904.00