You should definately chart for a few months to determine your LP.
It may not matter too much what time you get up, if the temperature in your home is regulated by a central heat and air unit. I don't know this to be fact, but its how it worked for me.
The first couple of months I charted I religiously made myself wake up at the same time every morning to take my temperature, and fall back asleep. After the first two months, I got a lot more lax about it. It really didn't seem to matter what time it was when I got up, as long as I had been asleep for at least 4 hours. My charts from those months looked the same as the first two, and sometimes I was waking up 4 or 5 hours later than other days.
When I entered it on fertility friend I always put in that it was the same time everyday, because it made my chart look crazy if I didn't. But, my charts were still very accurate.
So I say, don't worry about the time and just take it as soon as you can, and always, of course, before your feet hit the floor.