Dutch is talking about the day one patch coming with a bunch of new club home ground defaults - so it's probably ideal to start careers after getting the day one patch. But the patch also adds the option to let you change the ground you play a career match at pre-match, so you could just make your stadium any time and choose it.
You can also still set the custom ground as your home ground during career setup.
A few more things will be editable on day one.
There's also a general roster update as part of the day one patch - but if you make customisations to the on disk teams/players, you may not get those. If you play the game extensively pre-day one, then I'd recommend doing an academy reset when you get it, just so you have it all - including some updated likenesses on the England squad.