Does tilting the bike on a repair stand so the fork is vertical impede any repairs?
How your bike is positioned in a work stand should depend on the type of work you are going to perform.

Much of the time it seems more convenient to have the bike in the stand so it is titled forward with the front wheel a fair bit lower than the rear. Such as position prevents the front wheel from flopping over and puts many key components and systems such as both derailleurs, the rear brake, front crankset, handlebars and shifters at a good working height in relationship with each other.

This minimizes the amount of excessive bending and reaching required for many common tasks such as adjusting shifting, cleaning the drivetrain, replacing chains, wrapping handlebars etc.

If you are working on a less common task such as bleeding hydraulic brakes, replacing a bottom bracket, or installing a new bash guard on the down tube of a mountain bike; adjust the bike stand height and/or tilt appropriately to accommodate the specific task you need to perform. 

For something like bleeding a rear hydraulic brake this could mean lowering the overall height of the stand and rotating so the the front of the bike is tilted higher than the rear wheel to maintain an upward flow from the caliper to the brake levers. 

It can help to have the bike level when adjusting the saddle’s angle and setback but honestly it is usually easier to perform those tasks with the bike flat on the ground instead of in a stand.