It's hard to shine as a good support in Bronze because most everything you do needs to have some sort of follow up by your adc to make it a worth trade or not, especially when you are playing a traditional support, Janna or Sona can accomplish alot but its always WITH their adc, and in low ELO you have to depend on your adc to do the right thing. When you aren't with your adc your power and effectiveness is lowered, especially as a "traditional support".
To answer your question, a support can take it upon themselves to push a lane out, but it depends on why you're playing. If youre taking Sona solo to the bottom lane to thin out a pushing wave while something is going down somewhere else on the map, what you're doing is not optimal. A support with as slow of a push as that should only be taking care of a wave once its at your turret, otherwise you're isolated from your team struggling to thin wave when you could be in a team fight helping.
You'll waste more time by slapping the minions with wet noodles than if you were to let the wave do its thing. But now more into my are of expertise. If you're playing a much more aggressive support like I usually do, its not much of a problem at all. For example Zyra, Karma, or Lux could all clear a wave quickly and get back to the action without losing much time, but any support that can't power through a wave is essentially making the game 4v5 for a short time, and what's the trade off? The rest of your team is a man down, but don't worry, on the upside your support got maybe 10 cs and kept a turret up for the time it takes for another wave to come up, that's not much of an advantage to your team.
It's all pro's and con's, decision making, end results. If it's beneficial for you to be clearing that wave, by all means do it, if your time could be better spent somewhere else, do that. The answer to this question depends on the game, but rule of thumb, if it's going to be beneficial to your team, go for it, but if it leaves them weaker somewhere else on the map, its not worth it.
