starters - decant to step up?

I understand that, depending on the size of your starter, it's common to decant prior to pitching. my question is, when stepping up a starter should i chill, pour off the old DME mix, then add new DME? or simply add fresh DME mix to the existing starter without any decanting? i will likely need to step this thing up multiple times (culturing some JP dregs), so i was wondering what the best technique is for this. thanks!

Yes you will want to decant, just leaving the slurry and then make a new starter to add the yeast to. Making new wort gives the yeast all fresh sugars to consume. Also if you are limited in your starter vessel size it would be difficult to just add fresh wort to your starter with out decanting off the initial starter.

The only time I could see not decanting is if you were growing up a small volume in a large flask (say 100mls in a 1L flask) and didn't want to worry about dumping any of the yeast that you'd already grown up. You could boil a batch of 200mls of about 1.06 wort and after cooling, add that to your 100ml starter, giving you a final concentration of around 1.04. It would also ensure that you weren't accidentally decanting anything that hadn't flocculated yet, although a cold crash should knock most things out of solution. Otherwise, its just easier to decant and add fresh.