If you're going off of the estimated landing time of the inbound aircraft and assuming yours will be delayed, but the airline hasn't actually posted a delayed departure for your flight, then that's definitely risky.
If the airline has actually posted a delayed departure, then IME, it is very rare that the flight will depart earlier than that, even if the plane is at the gate (e.g. a maintenance delay). The airlines (especially AS) seem to be
very conservative about posting delays, I think because they know people will take it as the new departure time and be late to the gate). I'd make sure not to push it and get to the gate 2 minutes before door closure (because they sometimes do close the door a few minutes early to try to recover some of the delay time), but I regularly arrive at the gate ~20-25 minutes before the posted departure time and have never found the door closed early.
If flying an airline like AS and departing from an outstation (where recovery options using different aircraft are much slimmer), the risk of an unexpected aircraft swap is pretty minimal, and I've been known to plan to arrive at the airport late if the inbound is delayed, even without a posted departure delay. Those are the cases where I
at AS for being slow to post departure delays--like, really, the plane isn't even on the ground yet and we're supposed to start boarding in 5 minutes? Someone in Ops is asleep on the job in those cases...
BTW, I've noticed that DL seems to be
much more proactive about updating estimated departure times than other airlines and also tends to make aircraft swaps and gate changes more frequently--likely because of Delta's investment in better back-end IT (e.g., the Delta Nervous System) enabling them to react better to operational changes (something WN could learn from, as the entire country now knows). I'd be less likely to play it fast and loose with DL than with other carriers--if they aren't pushing back the departure time, there's probably a reason they aren't doing so, so be to the gate on-time.