I am always amazed at the variation in zone frost dates, Angie! We're both in zone 7, but you've got seedlings pushing up against the top of your containers and I've only got puny little guys, and am very glad to see 'em!
If you think you might need further protection for your tall seedlings, one solution might be to place a gallon-sized zip-lock type plastic baggie over your container, open-side down, and with holes in the top cut for ventilation/watering. I've done this, using sticky duct-tape if necessary to afffix the baggie to the container.
