Since SSR will rarely pick up a squawk from the ground (at my place, I'm lucky if it picks it up from 500'!), I would say not necessarily b4 you enter the active, but certainly yes please definately b4 you roll. I'm also ppl/me/imc used to work for Air Service Training (remember them?) at EGPT. Last 3 chks on the runway b4 roll were txpr - alt
strobes - on
rwy - clear.
Something I always stick to.
Consider this; A/c recovering on RAS to ILS at busy class G airfield, a/c 2 gets airborne to depart 3/4 upwind vfr, but not yet switched on txpr...OOPS AVOIDING ACTION!!!
How am I the radar controller supposed to know that the primary blip is remaining at 1000' if he's not wearing the squawk I gave him?
I give squawks for a reason (not just to ease my nerves), but anyway, doesn't the Good Book say somethin about
"Txpr-equipped a/c are to squawk 7000 + mode C from t/o to t/d unless instructed to squawk a discrete code issued by an ATSU, the exeptions being a/c in emergency (natch) and a/c remaining in the visual circuit (txpr to s'by)
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Once more unto the breach, Dear Friends...