My family's income is below the poverty line, and I'm very straightforward about that in a few of my essays. My rule is to only say it if they need to know, and if I say it, to say it upfront without a hint of shame. For me, there are a few situations where I do feel like they need to know -- it's something that has really shaped who I am.
My advice is, if you feel like you must talk about being poor, don't be too subtle about it. Make sure they know you're not ashamed. However, don't be negative about it either, or complain -- even though being poor is obviously a negative situation, they know that. You don't need to tell them how much it sucks. Instead, focus on how your family's situation has made you strong.