Terra has no tithe grade. Asking what is the tithe grade of Terra is like asking how much cash does a organized crime takes from its own Crime Lord.
You get the idea...
The major factor that plays here is that Terra needs all the defenders it can get (in case if Terra will be invaded directly). So if Terra sends of its troops and ships to other locations, it will be more exposed during an invasion.
Also keeping in mind that a notable portion of the people on Terra are in positions that would counteract being sent into the Guard or Navy, like being from the Navigator's Quarter or just being nobility in the first place, and of course the hordes of psykers sacrificed constantly, all the things that would get in the way of standard service
Well let's not forget that the Imperium of Man isn't exactly the most sane with its plans. Obviously the remains of what was the Solar Auxilia could be Terran though I doubt any of them are still alive. There might be some form of PDF and those that got put in the Sisters of Battle, the Custodes, etc, but I doubt they'd ever be used outside of Terra as Zixis said