One rule of thumb to keep in mind is that any monies for short-term goals such as this home purchase should not be invested in the stock market. Market volatility could present problems if you do not have a longer time horizon for such investments.
It is important to keep in mind that great risk exposure does not automatically guarantee greater rewards.
The key to keep in mind with IRAs is you're taking a long-term horizon. You've got to go in thinking there's going to be ups and there's going to be downs.