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 $170.87   10.349%   $188.5533   $188.55   $188.55   $188.60  
  Purchase #2 $219.44   3.336%   $226.7605   $226.76   $226.75   $226.80  
  Purchase #3 $128.82   10.586%   $142.4569   $142.46   $142.45   $142.50  
  Purchase #4 $103.67   9.826%   $113.8566   $113.86   $113.85   $113.90  
  Purchase #5 $78.47   4.487%   $81.9909   $81.99   $82.00   $82.00  
  Purchase #6 $42.99   8.180%   $46.5066   $46.51   $46.50   $46.50  
  Purchase #7 $168.47   13.122%   $190.5766   $190.58   $190.60   $190.60  
  7 purchase totals = $912.73       $990.7014   $990.71   $990.70   $990.90