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 $28.58   7.862%   $30.827   $30.83   $30.85   $30.80  
  Purchase #2 $26.89   11.861%   $30.0794   $30.08   $30.10   $30.10  
  Purchase #3 $142.69   10.579%   $157.7852   $157.79   $157.80   $157.80  
  Purchase #4 $119.83   9.567%   $131.2941   $131.29   $131.30   $131.30  
  Purchase #5 $117.66   3.356%   $121.6087   $121.61   $121.60   $121.60  
  Purchase #6 $139.41   8.840%   $151.7338   $151.73   $151.75   $151.70  
  Purchase #7 $20.26   3.684%   $21.0064   $21.01   $21.00   $21.00  
  7 purchase totals = $595.32       $644.3346   $644.34   $644.40   $644.30