Software rendering: Is it safe to assume that a window's surface's alpha channel is ignored?

I’m trying to figure out if it’s safe for me to make an optimization when blitting to an ARGB8888 or XRGB8888 window surface that would involve putting unpredictable values in the alpha channel. Will these pixels always display as fully opaque on all platforms regardless of the alpha value? Or, if this isn’t always in the case, is there something in the API to check or to enforce it? Or is this just not a safe optimization for me to make?