Years ago, I heard someone say that you should use tools like social media insofar as they support your overarching business goals. Everything I've seen since then has borne out the wisdom of that.
Yes, if possible, get the usernames you'll want now. That being said, services like Facebook and Google+ may only grant you a certain type of custom username after you've verified your business and/or website. Sometimes you also need a minimum follower count to qualify for such usernames.
As others have said, keep the accounts private if you're not ready to use them right away.
When you do feel ready to start public accounts, look through several peer companies' social media channels to see what they're using and how often. See what kind of posts do well for them. Some organizations post multiple tweets an hour, while others post a smaller number of articles, which they then promote multiple times and multiple ways throughout the day. On Facebook, I tend to see fewer posts, but it's extremely important that you share well-optimized content. Posts without photos and other types of information get very few impressions -- almost certainly due to the Facebook algorithm.
At the same time, keep in mind that these channels all support two-way conversations -- but nurturing that relationship takes time and attention. Twitter has a good guide for businesses that includes several success stories
, showing the different ways companies use the tool.
Lastly, if you end up deciding a social media presence doesn't offer much return on the time investment, you can always make a brief statement on your About page about the philosophical or strategic reasons for limiting communication channels. If you're doing so because of other things you're for
, and saying no to social media so you can say yes to something more important to you, such as better service in other areas, that may be worth noting.