Why would you fill subfloor cracks?

Because the flooring that is going over it is not carpet and pad (which is what I just ripped out). It is a "floating" floor. I doubt the cracks would make a difference but I am filling them just the same. Actually, I have discovered that Home Depot has larger containers of the wood putty (I think Minwax wood putty comes in a gallon container) and if they have it in stock I will try that. I had been using a small container of Elmer's wood putty.

If they are small enough to fill, it is not needed for a floating floor. Waste of time, but if it makes you happy.......oh well..

Are we talking a wood subfloor?

Be more concerned that your floor is "flat" to within the floor manufacturer's requirements for a successful installation.

Are you filling cracks in the sub-floor or gaps between boards?

Even plywood sub-floors need room for movement (not much, but they still need some room),

Plank type sub-floors are laid with gaps between the planks for the same reason.

If you fill the cracks the sub-floor can buckle.

Since movements downwards is usually restricted, the buckles move upwards creating bulges in the floor.

Gaps between boards. Oh great, now I have royally screwed up the subfloor by filling in the gaps. Things are not going well on this remodeling project. Not only are the gaps filled in, but the floor is not perfectly level. Are we looking at a new subfloor prior to installing the Mannington Adura Planks? Anyone know how much that costs in the midwest? This project has gotten well out of hand.

Level does not matter as much as flat.

Don't worry too much about what you have already filled.
It's not that big of a deal.
To echo the others flat is the most important thing for you.

When I said level I think I actually meant flat, unfortunately. I have stopped filling in cracks now, however. My fingers are numb from pulling out those inch-long staples in the plywood in the kitchen -- must be 75 of them per board (I use a little nail puller tool and hammer and pliers and it takes forever). The store said it would be easy to remove the kitchen vinyl and underlayment but I do NOT think so. Each sheet is glued down AND stapled with all those horrendously long staples (and sometimes nails as well). I wish I had left the kitchen alone for now and I hope at some point the result will be worth all this effort and expense.

It is absolutely amazing to me that flooring professionals actually think (thought) that glueing underlayment actually makes (made) sense.

If the added height of what is left will be no problem, then you may want to leave it alone and flatten what's left. There are very good ways to do that, but you would need consultation and guidance.

Good luck.

I can't leave it partially ripped out as there is height difference and also HUGE splintered wood edges all over the place, unfortunately. (And for some strange reason my cat grabs hold of pieces of wood sticking out and tries to rip them off with his mouth--and occasionally succeeds!)

At some point you have to cut your loses and just replace the entire sub-floor.