Ive done both.
Covering up one branch was
a) a hassle
b) seemed to trigger the whole plant into flowering. Though this may be coincidental, the branch did not show flower first, other branches did.

I've tried sexing clones, but a plant grows to sexual maturity in its own time. I dont think going 12/12 before the show gender speeds anything up, just gives them less light during veg and stressful signals.
People mix up flowering already sexually mature clones that you can flower when you want, and seedlings, that need to reach gender first.

Threw my clones away last night, because every plant showed gender before their clones did.

I keep my plants in as small pots as possible, 3 liter pots, until I see gender, then I repot as quickly as possible into bigger pots, as I downsize the amount of keepable plants.