He might use tactical nukes in Ukraine, it won't matter to the Ukrainians (in knuckling under to blackmail), but it will matter to NATO. Poisoning a significant portion of the world's grain supply will have consequences too, most of that grain goes to Turkey and Islamic countries in the middle east. It might be viewed as genocide, if millions starve and Putin will be to blame. He would piss off the Islamic world too and they are Russia's neighbors. Propaganda built on fact will be used to whip up anti Russian sentiment, if Starvation is the result of the use of Russian tactical nukes poisoning, both Ukrainian and Russian grain too from fall out. If he uses tactical nukes, he won't just use one, if he wants to have any effect on the battlefield he would need to a half dozen. I don't think the Russian troops will be very good at handling fall out, much of their equipment is probably old soviet shit and they have little training. Troops these days just don't bunch up, they are spread out and in vehicles designed for such things or in bunkers, trenches and foxholes.
So let's say he uses 10KT atomic warheads on medium range missiles fired from mobile launchers in Russia. IMHO he is most likely to do this if the Ukrainians are threatening to advance into Crimea and his army in the south is trapped. It would be the only place they would be useful, getting his troops to advance trough a mushroom cloud with little training and shitty equipment might be an issue for the Russians.
en.wikipedia.org