Is there any harm to installing the cleats a little bit closer than the manufacturer's nailing schedule - say 6". Note that I ordered more cleats than I actually need and don't mind spending a little extra time on this if it will make for a better install. I am installing 3/4" x 3 1/4" solid birch if it makes any difference to the above.

What is your opinion?

Don

No harm as long as the tongue is not splitting. Been doing it for years here.

Marco is correct. The only problem with nailing closer than the schedule recommends is tongue split and breakage. I have noticed, especially on brittle woods like maple and b. cherry, that placing fasteners too close together will cause the tongue to split/break/crack at times. One needs to be careful of this. It doesn't usually happen on oak unless you're nailing into a knot or hard spot. It is best to stay with the nailing schedule recommended. Strip says 10 to 12 inches oc. You can do 8"oc. Plank is 8" oc and could go a little closer but be careful of tongues breaking. Wide planks benefit from gluing as well as nailing.