I load my mtb on my car all year long, an average of 10 times per month, also travel long distances.Curious question: Why load it upright?
Do this for a number of years, both ways, upright and on the side, and you will find out that:
It's much easier to get in and out of the vehicle
It will wear and\or damage your vehicle interior and bike much less
You can secure your bike in place without it sliding while driving and potentially hitting you or braking soething inside your vehicle
You have a lot more cargo space
Last but not least, if you own a modern mtb bikes, with modern components, ie hydraulic disc brakes and hydraulic dropper seatpost, they will need much less maintenance, bleeding and or piston resets for lazy piston action etc
Less instances of bent derailer hanger
Etc
This might not apply to you if you use a road bike or carry your bike only once in a while