Nope, incorrect. Never start the upgrade with your final free worker, when you want to benefit from a special event like AT (saving you plenty of gems) is what I say.
That an event can save you time and gold, so be it. Gold can be required by raiding and wasting precious time by postpone an upgrade, in the vain hope that an event arrives that makes you a little time profit, is not what you should do. You should not wait and care for any event, except when you want to upgrade AT. By waiting a lot of time is wasted for nothing, don’t do that.
Most important is to have enough workers to keep upgrades going, when you can keep workers easily at work, it might be smart to buy another worker first. Having ‘enough’ workers (that is a personal feeling, for me 5 workers are enough, for others 20) is a must. 4 workers is minimum I would say.
That one and final worker you only save for situations where it saves you gems, so why wait when you don’t have the gems for upgrading AT? I only deliberately kept a worker free when I had enough gems to upgrade AT, despite the fact I was maxing defensive structures.
In fact I follow a complete different approach. As soon as I am not waiting for an alliance party, I always monitor my ongoing upgrades and know when the next worker is available. So I really don’t wait for an event when I don’t have intentions to upgrade AT. In fact I observe when the next busy worker comes available, but not with intention to profit from an event. Instead I use that info to make sure having the required gold available to start next upgrade as fast as possible. That’s how you save time and speed up upgrades, by not wasting time to collect the gold for putting a worker back on work.
When at that moment an event is active and it can make me profit a lot, I of course use that worker for whatever is in discount and costs less time. But for the rest I honestly don’t care. The primary goal is upgrading everything to max, you should never lose that goal out of sight.