Attribute Rolls

I remember the following methods of generating attributes being available in the D&D games I have played

1. Roll 3d6 and assign to attributes in the order rolled
2. Roll 3d6 and assign to attributes as desired
3. Roll 4d6, drop lowest, assign as desired

I think there is one more method that was in the 2nd edition PHB but I can't remember.

Anyway, has anyone ever done this:

Step 1: Roll 3d6 and total it
Step 2: Assign to an attribute of your choice BEFORE rolling the next one
Step 3: Roll 3d6 again, total it, assign to one of the remaining 5 attributes
etc
etc

I have always seen "assign as desired" done after you roll all six numbers. But has anyone rolled each one in series and assigned it before moving on? Have I been misreading it the whole time?

Just a flittering thought here on an idle Tuesday.