I think it depends on when you're brewing, if it's a couple hours from now I'd let it ride. If it's tomorrow, well, yeah a blowoff tube might be essential. A straight airlock will probably just get overrun by fermented stuff, not a big deal but might still create a mess.
This is the biggest reason why I've gotten in the habit of doing starters the morning of brew day, rather than a day before. I haven't noticed a significant difference in lag time between the two, the yeast are still ready and roaring for more. If I'm doing a lager, though, I almost always break out my 5L beaker to avoid any troubles, mostly because volumes are higher there too.