Miss Woo
When you don't know someone's time of birth, do you just type in 12pm in the Astrotheme thingy that brings up their chart?
From the thread title I'm guessing that your question relates to your pet dog?
Unless you ask in advance or happen to be present at the birth, it's virtually impossible to get an accurate birth time. You really have three choices, a chart set for Sunrise, a chart set for Noon or a chart set for Sunset. With absolutely no information on timing available, I'd suggest you use a Noon chart.
If you know that the pups were born between midnight and 6am, I'd actually suggest using a birth time of, say, 3am. That splits the difference between the two borders of the time-window during which you know your dog was born, whereas noon is very clearly outside of that window, and by definition, then, won't give you accurate results. Using a 3am chart won't necessarily be the perfect, flawless way to go, but it should give you results that are as close to "true" as you can reasonably expect in this situation. Just one person's opinion, of course!