But it depends where you spend. If you're spending at a place that instantly converts the bitcoin back into fiat, then you actually didn't help bitcoin much by buying with it. You just got it sold into US dollars and more US dollars used.
So I would recommend 1.) HODLing as the OP says, or 2.) spending at places that hold the bitcoin and don't instantly sell it.
I disagree. Even if you spend it at places who convert to EUR or USD,
as long as you also buy back the same amount of bitcoins you spend, you're not increasing selling pressure. And you help Bitcoin by increasing volume of Bitcoin consumer sales.
As one of the organizers of
Arnhem Bitcoincity, I have spoken quite some merchants who are saying: right now I still convert to euros, because I pay my costs and staff in euros. But I notice how I can spend bitcoins at more and more places. So, I will soon accept bitcoins and keep them, so I can spend them directly, and the whole conversion with euros won't be necessary anymore.
This whole thing with merchants converging to fiat is a
migration phase. As soon as there is a significant amount of places where they can spend their bitcoins, they'll get aware of the advantage of Bitcoin - not just accepting them (as a service to their customers or means of getting media exposure) but also for actually using it themselves.
Again: spending bitcoins and buying back the same amount is
always better than just hodling your coins and spending fiat. EVEN at places that convert to fiat.