That depends. (As does so much else in life.)

Do you have other domains active, anywhere? If so, consider parking the domains atop an existing site temporarily.

No other domains active anywhere? Then just leave them parked with the registrar, if the registrar will allow it. Just don't leave them parked there for too long - no sense in giving free advertising to the registrar, after all.

You might also just pick some really inexpensive host, get the cheapest account for a month, and park the domains on that account while you search for a permanent home. (This could also be a good way to do some preliminary research on a host - use the 30 days to test their support, explore their user documentation, check their uptime and speed, see if they're a good fit for you. If you have the domains pointing to a blank page for a month, that's a month where you're not trying to split your attention between testing the host and developing your site.)