I was taught growing up that it was good for me to pray about anything and everything because God cares not just about the "big stuff" but about the "small stuff," too. When I was in seminary, though, I had a professor pose the same question that I did above: Is my need for a parking space really on God's radar? My kneejerk reaction was to respond with an emphatic "yes!," but then my professor asked a poignant follow up question: "What is someone else is praying for that same thing, and what if there's only one parking spot?"
As good questions do, that question forced me to ask more questions: What made my prayer any more important than another person's prayer? Would God grant my prayer if it meant letting another person down? Does God pick favorites when prayer requests conflict with each other?
Sure, it seems silly to get so worked up over a parking space, but Christians pray prayers like my request for a parking space all of the time. Does God really care which team wins a football game? Does God change the traffic lights green for you so that you can get to work on time (potentially making others late)? Or, more seriously, does God intervene in lawsuits, political elections, or military conflicts?
Your personal theology and political leanings will probably influence your answers to those questions, but if I'm honest with myself I have to say, "I don't know." Do all of the things I've mentioned matter to God? I believe so, mainly because God's beloved children are involved in all of them. But beyond that, I have no idea how God sorts out all of the potential conflicts of interest that God's love for us inevitably causes. I do know, though, that God desperately seeks to be in relationship with us and that such relationships are only possible through communication.
So, should we stop praying over the "small stuff?" I think it depends upon our reasons for doing so. If we expect God to favor us above the rest of God's children, we may be setting ourselves up for disappointment. But if we're simply seeking to be in constant communication and relationship with the God who lovingly created us, I don't think that there's any matter too "small" for us to pray about.
In short, I doubt an angel will threaten to smite us for praying about silly stuff, but I have no doubt that God will be happy to hear from us.