For me, it depends on what they are acclimated to. My plumeria sit outside all winter in central FL. When it is projected to go below 35, I gather them together under a roof, cover them with a frost blanket, and put a couple of 100 watt shop lights in the middle of the group. I have a remote thermometer there and often the temps under the blanket will dip into the mid 30's. If the temps are expected to be high 30's or above, they sit outside in their pots without cover. They do fine. I haven't lost one doing it this way. Right now they all have new claws on every tip. HOWEVER, this is what I have done with them since I rooted them. I would hesitate to put a new seedling or new plant from a warm winter site right into this environment.

But I believe the general consensus is to protect them when temps drop below 50 overnight.

IIRC, most need protection under 40, but I would keep any reds and rainbows inside below 50, as Mike said.

Supposedly whites and yellows are most cold-tolerant (to upper 30's or so), except Singapores, which tend to get black tip if exposed to around 40. Reds and rainbows are said to be most cold-sensitive, so I keep all my precious ones indoors as soon as the temps start getting down to low 40's at night. As much money as most of us spend on these, I guess better safe at 50 degrees than sorry at 40!

I had my plumerias out since April and they saw a night of 39F without any problems. One of them even had an inflo. during that cold!
As long as it warms up during the day significantly and it isnt raining or wet, it should be fine. But if its getting to the low 40s every night then the plant will go dormant if it isnt already. Too much prolonged cold will make it rot.
But if your getting worried about them then just cover them up! It cant hurt them and you'll feel better knowing that its a little protected from the cold.

Good luck!
-Alex

I should have added that it is important to know how long temps will stay below a certain threshold. I check hour by hour forecasts. We have had nights where it is below freezing for 10-12 hours and my method of covering with lights for warmth works well. Those nights I leave them uncovered with temps in the high 30's, those lows are only for 1-3 hours, usually just before dawn. If the temps are that low for 6+ hours, I will cover them as usual.

what does covering do? seems like they would need to be generating their own heat for that to make a difference.

40 deg is magic # for me in tx gulf coast. Stick in unheated garage w/o additional protection and w/100% survival

Since I live in the bottom of a cool valley in northern Utah where the climate is dry, it's very common for night time temperatures to dip down into the low 40's even in the summer months. I never worry about bringing my plumeria in (about 20 of them) until the temperatures drop into the mid 30's, and they seem fine. Maybe they have become accustom to my climate.