That's a good question. I'm not sure that I can offer any kind of definite answer, but maybe one more stream of thought can offer some clarity.
First off, congratulations on your acceptance! Very exciting. If you know for a fact that you are going to attend USC, you are in a great position.
As for your potential scholarship, it can be a tricky spot to navigate I suppose. One question that I have for you is whether it is a departmental merit scholarship or a university-wide scholarship? It's possible that if USC has you locked in, they may view the merit scholarship as a means to entice an additional student whose intentions they are less certain of but want just as badly. Scholarships can be great recruiting tools after all. On the flip side of that, it's possible they may be more likely to offer the scholarship to someone they know is committed to the program. You would like to think that these scholarships are objective and based entirely on achievement and fit, but it is difficult to completely neutralize one's subjective bias, even a subconscious bias.
With that being said, I see no harm in waiting to officially commit to USC until after the scholarship is determined. It is not as though you are going to lose your spot, unless for some reason USC does not abide by the April 15th consideration (which I'm pretty sure they do.) I think it is more likely that the school would use the merit scholarship as a recruiting method.... not that they do that