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 $13.30   4.642%   $13.9174   $13.92   $13.90   $13.90  
  Purchase #2 $29.75   4.318%   $31.0346   $31.03   $31.05   $31.00  
  Purchase #3 $81.42   9.335%   $89.0206   $89.02   $89.00   $89.00  
  Purchase #4 $215.77   7.791%   $232.5806   $232.58   $232.60   $232.60  
  Purchase #5 $215.65   7.172%   $231.1164   $231.12   $231.10   $231.10  
  Purchase #6 $76.45   10.615%   $84.5652   $84.57   $84.55   $84.60  
  Purchase #7 $45.44   12.770%   $51.2427   $51.24   $51.25   $51.20  
  7 purchase totals = $677.78       $733.4775   $733.48   $733.45   $733.40