Here is an interesting problem: how do you import a stack of leads and evenly distribute them between several reps? There is no inherent function within salesforce to accommodate this feat. But there is a way to do it. Here is one good way.
Essentially, you put an auto number on the lead record that assigns a new number to each new lead as they are created (or imported). To evenly distribute the leads between several reps, you simply divide that lead number by the number of reps and use the remainder +1 to assign each lead to one of the reps.
I'll use three reps in my example. If the lead number (the one we automatically generated) is divided by three and you just examine the remainder, you'll see this progression:
Lead Number Function Result Remainder Remainder+1 (Assignment)
1.......................... 1/3........... 0......... 1.................... 2
2.......................... 2/3........... 0......... 2................... 3
3.......................... 3/3........... 0......... 0................... 1
4.......................... 4/3........... 1.......... 1................... 2
5.......................... 5/3........... 1.......... 2.................. 3
6.......................... 6/3........... 2......... 0................... 1
Then just use assignment rules to assign the lead based on the result: just assign all the "1s" to the first rep, all "2s" to the second rep, and all the "3s" to the third rep.
The way I did this was to create two new fields. The auto generated number field called "Lead Number" and a formula field called "Assignment."
The formula I used for the Assignment field was this:
MOD( Value(Lead_Number__c) , 3 ) + 1
MOD gives you the remainder of a division, so you can see that I divided Lead Number by 3. The "Value" function takes Lead Number, which is actually a text field, and turns it into a real number. Then we add 1 so that a 0 is not returned.
Then create your lead assignment rule called "Round Robin" with 3 rule entries. The first entry will assign all leads with an Assignment field = 1 to the first rep, and so on for the second two rule entries.
If you have more reps, just divide by that bigger number and create the right number of rule entries.