It is better to launch your forum with some content rather than no content. Buying posting package is one of the best ways to build content early on. If you have funds, you can also hire paid posters. I have seen a lot of forums using these methods to build activities before officially launching.
Honestly what's important is that you open with people who want to be there on day one who aren't there because they are staff or paid to post on the board. I seen so many who think they can just open with no one or just staff and expect to flourish and it just doesn't happen. I don't think it is a problem to use post package services to help with this but it can't act as a substitute for a people genuinely interested in being there. With boards I owned outright, I had integrated myself into a network before hand which helped with not only getting people willing to be with me on day one but also anything else the community needed or wanted. I only had to show my needs and it always seemed like someone was always ready and willing to step up to fill that need.
The act of building a forum is far harder then many think because there is so much work that needs to get done before you even get to putting up and building an actual board. It's no surprise that so few boards that get built actually last long enough to even get established. It is however as equally a rewarding endeavor if you manage to pull it off.
I think it's a great concept to have content for when your community opens and it will look as if people are more active. This is more welcoming at the opening/beginning of a website/community/etc. My biggest recommendation or consideration is that I personally wouldn't request posts too early. Meaning if I Intend on opening in February, I shouldn't be requesting packages in December. I would want the packages within days of opening. The closest to opening the better. These users could also end up being beta testers and happen to come across little bugs which is also helpful!
I think requesting posting packages definitely helps! Especially if it's a new community needing some engaging content. Heck even if you have a well established community, requesting packages from time to time can help boost activity.