Ideally you want a well contained growspace.
Light leaking in or out when not necessary is always going to be a negative. However depending to what extent makes a huge difference. With cheaper grow tents the stitching will not be as tight and the light may be slightly visible through the fascia zips. Cheaper tents are normally good for 3/4 runs.
The best tent I have seen on the market is the Green Qube. ( its on the wish list for sure)
Check out there promo video below.
The aim is to act as GOD to your girls and control your grow enviroment to the best of your ability.