Sometimes yes, you can measure from the top with extenders. Some tubes and circuits act differently though when using an extender.
Converter tubes sometimes won't oscillate due to extra capacitance of the extenders.
Tubes using shields present a problem too.
I use extenders mostly for quick checks on a single tube or two.
For extensive testing I work with the chassis upside down.
If a reading is off, you then need to follow previous parts on the schematic and you can do this only upside down
I often draw diagrams showing chassis upside down with socket orientation relative to the other major parts, then transfer voltage measurements from the provided charts to the correct pin. No confusion this way.