@haqian
Not if it is a numeric field that you are using. If it is text, then yes, you could keep them.
Although you could use the Text function to format the number, the question would be, how many leading zeros. If you know it will always be, say, 4 leading zeros, then you could use : Text(yourfield, "00000") That would give you 4 leading zeros and a digit.
But, if this is random (one could be 2 leading zeros another 5 and so on) and the length for the number is variable, then you will need to store as a string.
I hope this is helpful for you.