That may work in the short term, but long term you'll have problems! Besides, you could under fill it/over fill it and have immediate problems.
Best to bite the bullet and do it right from the get go. If you take short cuts you'll likely regret it down the road. Doing it the correct way now will save you aggravation. I've never heard anyone say "Darn, you know, I wish I had taken some short cuts on my flooring years ago"