The L. argenteum is typically grown as a single trunk tree rather than a shrub, and this is what it prefers to grow as, so unless you have a particular reason to prune it, I would leave the central leader alone. If you want to encourage bushier growth on side branches, you can certainly tip pinch them. The other shrubby Leucadendrons all benefit from tip pinching to make them less leggy, and/or bushier, but it sounds like you have planted them in less than full sun which they need to stay more compact. If they are reaching for the light, even regular tip pinching will not keep them more compact. You can pretty much tip pinch at any season, I usually wait until the fresh growth has hardened off abit myself.