Also that it may take time for the equipment (or even the gear itself) to 'settle' into their ideal locations.
Like the fact you might not appreciate that dasblinkenlites on the front of your equipment turn out to be a huge peripheral vision distraction when sitting across the room from the stuff. So you'd need to move them up/down to get rid of the distraction, or hide them behind a panel. Or that the tray for the disc player(s) are too high or low to let your kids/spouse load them without breaking them... again.
So while something might look nifty draw out ahead of time in Visio or some other rack planner, it might not turn out to be useful laid out that way.
It's true a custom installer would be thinking about all this for you and taking all that into account. along with making all wires look pretty. Just expect to pay an awful lot for that... And still end up having to move things around.
So once you really know where everything work best, then you just replace any cables as needed. Yes, you'll end up with extras. You'd want to order more than you need anyway. Keep those on-hand for the day when you have to replace something. Because, inevitably, the connectors on the backs of the replacements won't be in the same places...