My feeling is that if your plants are brought on early then you have to put them out whilst the weather is still unsuitable. I just grow our plants 'as nature intended' and grown hard.

Depends what you are growing.
I use grow lights for my onion seedlings, it does make them grow quicker, but unless you are exhibiting them then it's not really worth it in my opinion.

Far more important is keeping uniform warmth in the greenhouse, ie, never let the temperature fall below 50'f when your plants/seedlings are growing.