You should start flowering when you want the plant to start flowering.
Do you want a large plant that will yield more but require more room/light/everything else?
Or do you want a smaller plant that will fit into a smaller area?
The 5x5 doesnt isnt really the determining factor, you left out the last most important dimension.
And you should really try to maintain equal height prior to flowering so that the plants all get as equal light as possible. If one is 16" and the rest are between 12-16" you should be ok. If the rest are much smaller I would train the tallest one by topping, super cropping or LSTing so they even out.
If they are all close to 16", you are going to run into some space issues when they fill out during flowering. You didnt mention you lighting situation in wattage. I would have 1000w HPS in that area, especially if you hope to get the best results from nine plants. You might consider flowering only 4 or 5. Take cuttings and start 4 or 5 of those for when the others finish.