There is a fuel limitation in the MU-2 based on the ZFW cg of the aircraft, if it is nose heavy then you can only put fuel in the mains or mains and outers. You can find this information in section 7 of the manual i posted in this forum. If there is no fueling limitation applied then you should fill the mains first, then the outers, then the tips. There can be several reasons for this here are a couple major one.
1) if a fuel transfer system failure occurs the resulting imbalance roll moment is greatly increased if all the fuel is out on the tips.
2) In the event of a bleed system failure or leak in the tip tanks you may not be able to transfer fuel from the tip tank(s) Proceed to 1.
If you use the manual and go through the flight planning for section, you can get proper fuel numbers. I put a request into simbrief(Navigraph) to add the MU-2.
Below is an image of my weight and balance form for the MU2. For Flight planning I plan an average climb TAS of about 185knots and 670pph for climb (this is based on the book values, the TOGA MU-2 fuel burn is about 14% too low). for cruise the flight planning tables lay it out quite nicely, and if you are using my mod, your speeds will be spot on but the fuel flows are still 14% too low.