I don't think anyone recommends getting to temp slowly in something like a blade because there are temperatures between your presoak temp and your austenitizing temp that are not good to dwell at. You want to get to temp as quickly as possible without overheating spots. This is one reason why molten salt works so well.
People dealing with large tool and die type work have to compromise because of other issues that don't apply to us. So there are things they do that we don't.
Best practice, with ovens like this, is to preheat in one oven then charge the work directly into the hardening oven already at full temp. Due to the fact that knives are relatively thin and (compared to a stamping die) uniform, the issues of distortion and uneven soak time are largely moot in a knife and the presoak can be skipped altogether. It's a good practice to preheat, but if you only have one oven, and your work piece is a knife blade, it's probably better to skip it. Particularly in the oven we're discussing.
I suggest preheating the oven hotter than you intend to use it before putting the work in because this particular oven has exposed coils that are inches away from your work, so time spent getting back up to temp will overheat areas of your knife. This isn't theory, I discovered this problem with my own EH oven through testing and developed this work around to avoid the problem.
