Initially Nimrod used to wander around the longitudinal axis by almost 2 deg during an astro shot. Compared with the 0.1 degree wander in a Vulcan the Nimrod was a pig.
I believe the initial mod to increase the stability was to fit VHF aerials on the tail. Later, when the mod was formalised, the larger purpose designed strakes were fitted.
As for timinng, the strakes were fitted well after it was introduced into service and only as part of the Op Corporate AAR Mods in 1982.