The boiler needs to dissipate heat, so it's unkind to block off all the outlets. If it gets too hot, you will get a "fan-rush" in attempt to cool down. I think it's a bit of a non-sense to say don't close off vents because they're unlikely to know how many you have, so I think it's cautionary advice. Also, what is meant by "closing-off"? It may mean shutting completely rather than closing slightly (hope that makes sense). The important thing is to leave enough opportunity for the boiler to shed its heat (and not melt).