sjN, thanks for reposting that guideline from Fusion - my favorite too.
I have closely observed plants at daytime highs not reaching 60 F, and it would be rare to see any evidence of leaf growth, opening of flowers, or setting of fruit. OTOH many cool tolerant varieties will grow, flower and set and grow fruit just fine at any point between 60 and 70 F. Some other varieties really don't do much fruit growth unless highs are 70 F or above, and the nighttime lows of 60 F make a big difference for them as well. I started paying attention to the 60 F night after AKMark mentioned it, and I do think it has a marked effect on the rate of fruit growth and ripening across the board.
I used to consider 48 F as the nighttime low where I should protect the young plants. However being a bit more careless I found that plants showed no stress with nights down to 45 F, as long as the soil wasn't too wet, foliage also dry, and were not suffering additional stress ie from just being transplanted.
Fusion's advice about transplanting when you have a couple of warm days forecast was the best ever. The plants get comfortable in 2-3 warm days, and are then able to handle some cool temperature stress without showing it.
Also the research which Vladimir brought forward, about cold tolerance being better when dry vs wet has also proved correct for me - I did some experiments on 'extra' seedlings that were outdoors, and sailed through the cold nights with no damage if I didn't water them, vs. those that I watered were a mess of dead and fungally infected leaves after the same night.
Since my greenhouse isn't heated, I aim for night lows of 50 F as optimal for putting out seedlings or young plants. Warmer is fine, but anything below 45 F, is getting into the danger zone and I will cover them to prevent going lower than that. I will often open the vents if daytime temps get above 70 F, try to keep it around 70 -75 F and will close if it drops below 65 F.