Let them all grow, and plant them out as a clump.

Interesting Donn ... You have always been a 'grow it in clumps' grass seed sower and I have aways separated my grass seedlings into individual little plants & transplanted each into its own 'cell'. Either method works! I think the difference is either a guy vs girl thing or ..... one of us is much more of a 'control freak' (that would be me :o)
Tho ... One thing I have found is, while growing the seedlings under lights, they really benefit from a couple of 'haircuts'. It really makes them bulk up both, above & below ground.

I have one plant of Festuca mairei and I love it!!
Hmmm ..... wish I had sown some Nasella tenuissima this year :o( A.

I'll isolate seedlings if I'm looking for certain characteristics in a grass. For example, when I sow Blue Fescue, or Miscanthus 'New Hybrids,' which is a blend.