Another technique is to mimic that used by Napoleonic battalions (and probably as far back as Greek phalanxes if not further). 

Frontage is generally maintained constant, and as a man falls (in any rank except the back) he is replaced by the man directly behind him stepping forward. The back rank is shuffled by the NCO's to ensure a few men at the extreme of each flank, and otherwise to fill in evenly. 

The frontage is only reduced when the back rank falls below pre-specified densities. Likewise, when the back rank is overfull the extras first start filling in an additional rank from both flanks in, and then the frontage is increased.

When changing frontage, I suggest having your bots file out from the back rank to both flanks when increasing frontage, and filing in from both flanks to the back rank when reducing frontage.

If I am correct in inferring that you are looking for a "military" impression, and having your bot organizations look like phalanxes, I believe this ordered re-arrangement is a better way to achieve that end.

**Update**:  
One simple way to manage the back row is to divide the back-row units into three squads: one on each flank and one in the centre. Depending on whether the frontage is odd or even, and whether the number of back-row units is congruent to 0,1,or 2 mod 3, there are exactly six cases to manage.

As an enhancement to the above, consider spacing the last unit(s) of each back-row squad once the fill drops below a threshold, like this:  
       xxx.x....x.xxx.x....x.xxx  
or this:  
       xx.x.x...x.xxx.x...x.x.xx  
A bit more work, for an even better appearance.