My client has about 180k subscribers with active email addresses. This is total. Very low amount compared to bigger organizations. I'm creating their IP warming plan and they will NEVER even reach the top threshold limits for sends. They will be way below the threshold each week. So, my question is, "is IP warming really even necessary for this particular client that doesn't send very many emails?"
A general rule of thumb is :
20,000 emails, per domain, per IP, per Day. The per domain element should consider grouped providers like Hotmail, Outlook.com etc
AOL is a bit different and best started at 5000 emails per day.
Always try and start your sends to the most active engaged customers that you know - it might always be possible thought.
You really want to do an gradual IP warmup so that you can clear out any bounces etc. Always start small and build up volume. If you stick to these rules then you should get a good deliverability. Also ensure that your DKIM and SAP authenticated domains are set up correctly.
You will indeed need to do it in order to get a positive sender reputation, as by these numbers the client is not in the low-amount bracket anymore.
To accurately define the amount of Emails-per-domain during the warm-up period is something you can discuss with Salesforce Deliverability Team by creating a MC Support Case.
With these topics its generally a good idea to follow best practice guidance, as getting out of negative reputation is both time consuming and costly.