Unless you are planning a control panel of "olden" days with a track diagram and toggle switches protruding from it at given locations, many of the panels now seen on these pages are just locations where power and wiring start, with the biggest concerns being large enough to handle power supplies, heat and access for repairs or modifications. So if yours is to be of the newer style, the best location would be on either side of your widest aisle. Then, you would put your toggle switches (or whatever you decide to control your powered on-off sidings) right on the railroad fascia near the siding location.
On the other hand, if you choose the "olden" style panel, Dave's location is probably best because you will probably spend a lot more time building and disassembling trains in the yard and moving engines in/out of the roundhouse than anything else. The current versions of "olden" style panels generally conceal the power supplies and wiring under an angled panel top where the user sits next to it.
Chuck
