Jamil says over 5% and you're pushing it... I like to save the yeast from a smaller beer (like a pale ale, brown ale, irish red), rinse it, and divide into about 4 containers. Then I build starters for my beers from these smaller cultures, sometimes having to step them up. When I get to the last container, I make a smaller beer again to restock.
You'll probably get people who will say, "Your beer will be fine if you use the 7% yeast," and you're right - I'm sure you'll make beer from it. But will it be as good as it could? That's the reasoning I use EVERY time I'm tempted to do something lazy in brewing, like not making a starter or pitching onto a yeast cake. I mean, we put so much time, effort, and money into crafting a perfect recipe with just the right balance of this and that malt and hops. And then we think it's okay to get lazy with the most important ingredient?
You can practically throw any balance of malt and hops in a beer and it'll turn out fine so long as you take excellent care of your yeast. So pitch that new packet of yeast and make yourself a small beer for rinsing/storing!