I check my temp vaginally because if I do it orally my temps are just crazy, they make no sense and jump around all over the place, since I switched to vaginal they have calmed and I can usually see a pretty clear shift.
I think that most everyone usually starts with oral temps, and then if they are still inconsistent or very erratic, then they move onto vaginal temps. my temps orally were quite jumpy actually, but were still okay enough to show a clear shift when I ovulated, so it was good enough. Had I not conceived so quickly, I may have very well switched given time though.
I temp vaginally as well;. My temps were wildly erratic and jumpy showing HUGE shifts in temps. Part of the issue is that I'm a mouth-breather at night because of my sinus and allergy problems. Also I have these bony growths under my tongue called Torus mandibularis that seems to prevent good placement of a thermometer. Once I switched to vaginal temps, they remarkably stabilized and I could see a pattern.
If you can interpret your chart with oral temps there isn't really any need to do it vaginally. It's normally only if temps are very erratic that vaginal temps can help make things clearer.