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 $29.58   9.806%   $32.4806   $32.48   $32.50   $32.50  
  Purchase #2 $190.93   13.784%   $217.2478   $217.25   $217.25   $217.20  
  Purchase #3 $139.72   13.623%   $158.7541   $158.75   $158.75   $158.80  
  Purchase #4 $238.31   13.724%   $271.0157   $271.02   $271.00   $271.00  
  Purchase #5 $146.73   10.480%   $162.1073   $162.11   $162.10   $162.10  
  Purchase #6 $172.66   7.148%   $185.0017   $185.00   $185.00   $185.00  
  Purchase #7 $46.43   7.289%   $49.8143   $49.81   $49.80   $49.80  
  7 purchase totals = $964.36       $1,076.4215   $1,076.42   $1,076.40   $1,076.40