The time zone settings that are set inside any sequence will also be the time zone the sequence will follow to send the emails.
Of course, email sends will also follow any other settings you have set, like 'time of day' or 'day of week'.
Your sequences will default (unless changed inside the sequence settings) to whatever you have your company's Global Timezone Settings set to.
If you are sending one-on-one emails from the Contact View and scheduling to send later using the 'Send Later' functionality, it will also send later at the scheduled time on the basis of the Account's Default Timezone.
From a VA or Assistant perspective, they shouldn't need to make any extreme calculations to factor in what time to send to, as it can simply be denoted by the time your company's global settings are set to (it will not send out based on the user's local time zone and will instead follow the timezone settings).