That has changed substantially, and it has not changed everywhere, which is now the real trap. Most major operators accept an e-ticket with a QR code. Some smaller and local operators still require a printed boarding pass. Assuming you have the first when you have the second is how people miss boats.
The one thing to do
Read your confirmation email properly. It says which of the two you have, usually in as many words: either a boarding pass you can show on a phone, or an instruction to collect printed tickets from a named office.
That is the whole discipline. Everything else on this page follows from it.
If the email is ambiguous, treat it as paper and go early. The cost of arriving an hour before an unnecessary collection is an hour; the cost of the reverse is the sailing.
If you have an e-ticket
Show the QR code at the ramp or at the gate and it is scanned. Carry it offline as well as in your inbox, because port wifi is not to be relied on and phone signal at some island quays is poor.
Bring photo identification. It is asked for, and it is required if you booked any discounted fare.
If you need to collect paper
Collection points are named in your booking confirmation, usually an agency at or near the port, and they open a while before sailing rather than all day.
Two practical notes. Allow more time than the collection itself needs, because at a busy port in August the office has a queue. And be certain of which office: on the bigger quays there are several agencies and only one of them holds your booking.
Piraeus is its own problem
Worth its own section. Piraeus is very large, the gates are numbered and far apart, and the walk between two of them with luggage is longer than most people expect.
Find out your gate before you arrive rather than at the entrance, and if you are also collecting paper tickets, do that first and then walk. Airport transfers in Greece covers getting to Piraeus from the airport, which is the leg people underestimate at the other end.
Vehicles are a separate matter
If you are taking a car, the vehicle has its own booking and its own boarding process, and you queue in a lane rather than walking on. Confirm that the vehicle space is on the same booking as the passengers rather than assuming.
Taking a car on a Greek ferry covers that side in full.
When the sailing does not happen
Separate from ticket format and worth knowing. Cancellations for weather are a normal part of the system rather than an exception, and what happens to your ticket depends on the operator and the reason.
When a ferry is cancelled sets out the mechanism, and if you miss a ferry covers the case where the fault is yours, which is treated very differently.
Where this leaves you
Read the confirmation email and find out which kind of ticket you have. If it is an e-ticket, save the QR code offline and bring identification. If it is paper, note the collection office by name and go earlier than you think, particularly at Piraeus. And if you cannot tell which it is, assume paper.
Quick answers
Do you need to collect Greek ferry tickets in person?
Increasingly not. Most major operators now accept an e-ticket with a QR code scanned at the ramp. Some smaller and local operators still require a printed boarding pass collected from an office at the port, and your booking confirmation says which applies.
What happens if you turn up with only a confirmation email?
If the operator requires paper, a confirmation email is not a ticket and you can be refused boarding. That is why the email itself tells you which you have, and why an ambiguous one should be treated as requiring collection.
Where do you collect ferry tickets?
At the collection point named in your booking confirmation, usually an agency at or near the port, which opens a while before sailing rather than all day. On large quays there are several agencies, so identify the right one before you arrive.
How early should you get to Piraeus?
Earlier than you think. It is a very large port with numbered gates far apart, and walking between them with luggage takes real time. Find your gate before you arrive, and if you also need to collect paper tickets, do that first.