For the last 5 years I've been starting seeds using the baggie method which is like instant gratification.  I love being able to start seeds and have green shoots with leaves within a week.  The only problem is I've found myself pushing when to start the seeds.  Last winter I was so anxious to grow anything that I started some sunflowers right after Christmas.  I'm not sure what possesed me to do this, but I did.  I ended up transplanting them to the largest peat pots I could find and they were a good 2' plus tall by the time it was warm enough to bring them outside.

This year I was more restrained...somewhat.  My tallest plants are the zinnias and they're only 6" to 8" tall right now.

I would strongly recomment using peat pots for your seedlings.  This way you can just dig a hole and drop the whole pot in without having to disturb the roots.  Plus they help retain moisture while they get established out doors.