Well, I have been using photoshop for quite some time...and everytime I save the images... I choose "interlaced" but I thought that was the best way to save images... until I was messing around with images, by saving the original PNG's as BMP and discovered that the images that I pieced together with photoshop are now "exposed" and looks messy.
I just go Save as... and then save it as a .png. It depends on the project though. Most things are within the 800*800 pixel range, so it wouldn't matter too much, unless you have many large layers.
Also depends on the type of image. Transparent looking images go better with png then say gif for example. Png also uses more resources than gif though so be aware.