Perhaps he should be more careful, in fact. The various tactical voting groups have been a bit more thorough in checking who can beat the SNP in various seats. At the most recent survey, in fact, the Conservatives were best placed to beat the SNP in something like half a dozen seats. Vote Labour there and you might well let the SNP in! (Obviously this leaves something like 40-odd seats where Labour are the best bet for a tactical anti-SNP vote; I think the Lib Dems still have a chance in a handful too).
Tactical voting against the SNP is worth a try in Scotland, but probably won't work. Sadly, not enough people -- left and right -- will be able to tick a box for their biggest rivals. British Politics is getting too tribal. Or perhaps it always was, and I've only just noticed.
Here's hoping that people will listen to Tebbit on this, all the same.