I don't believe it's particularly bad as long as your heart is healthy.
The risk with viagra is provoking hypotensive shock through over-stimulation of the gCMP pathway and over-relaxation of the vaso-endothelium i.e. it makes blood vessels bigger and your heart becomes incapable of perfusing your brain with blood. Only in overdose though, or when combined with other drugs such as nitrates.
The half life is 4 hours or so, according to the pharmaceutical company that makes it. So it should be entirely out of your system by evening. If you're really worried though, monitoring your blood pressure to see whether it is too low would be a reasonable precaution.
Below 90 systolic is abnormally low fyi.