I've got a number of friends that compete strongman, some of them just had their debut competition, and one of them recently got sponsored. Something you have to keep in mind is that the overhead work needs to be lifted off the floor. The individual competitions vary based on the implement used and the standard weights. The implements might be Apollon's Wheels (train axle), fat grip dumbbells, barbell, log, etc. From there:
- Clean the bar/implement to the shoulders (either continental clean, or power clean)
- Press over head any way you can (some competitions are for weight, others are for reps with a given weight)
It's important to practice the full event. However, if you are focusing on just the overhead pressing to put in work and get stronger, working from a rack lets you put more effort into the press movement itself. Just remember to give yourself practice with the movement from the floor.
To directly answer your question though, the importance of the negative depends on whether the lift is for the heaviest single, or the most reps.
- You must be able to control the implement in free space. Do not be tempted to slide the bar along the uprights in the power rack. (the set up you described sounded like that could be a possibility if you aren't careful)
- If your competition is for reps, don't let the bar touch the rack until your set is over.
- Practice with the implement you'll be using in your next competition.