Some plants are long day plants -- they require a certain number of days longer than a certain number of hours.
Some plants are long night plants -- they need nights that are longer than a certain number of hours.
See this for more details: http://www.biology-pages.info/P/Photoperiodism.html
Other plants are day neutral. They bloom when triggered by other stimuli such as it being warm enough.
If you don't care if they bloom or not, then you can give them 24 hour light. They should grow somewhat faster.
However, you may need to check what triggers them into dormancy. E.g. Spruce start going into dormancy when days get shorter than about 10 hours (Varies by species, and provenance) Reforestation growers trick them into going dormant in August with black tarps in mid afternoon. They can then plant them with minimal stress, and they take off in the spring.