Quick answer · practical
Do you need a guide for Sepilok? It depends which half of the trip.
The short answer
For Sepilok's three centres: no. They're self-guided, well-signed, and a ten-minute walk apart — turn up, pay, wander. For everything beyond the boardwalks — the river, the night walk, Turtle Island: yes, and not as a formality. Guides are built into those experiences, and the guide's eyes are most of what you're paying for.
Where you genuinely don't need one
The Orangutan Centre, the Sun Bear Centre and the RDC in daylight are the most independent-traveller-friendly wildlife sites in Borneo: fixed feeding times, clear boardwalks, staff on hand, everything within walking distance of the reserve's lodges. A guide here adds context and comfort — species stories, the best standing spots, zero logistics — which is genuinely pleasant and genuinely optional. If your budget has one splurge in it, don't spend it here.
Where a guide is the experience
On the Kinabatangan, guiding isn't an add-on you decide about — every cruise comes crewed, and the difference between guides is the difference between trips. A sharp guide reads flattened elephant grass, hears a black hornbill before it shows, and picks a tarsier's eyeshine out of a dark wall of leaves at thirty metres. The same holds for the RDC night walk (guided by design, booked by 17:00) and Selingan, where rangers run the entire night. You cannot buy better wildlife odds with money alone anywhere on this circuit — but you can with a better guide, which is why "wildlife is our priority" is the single most useful sentence to put in an enquiry.
The honest middle: Gomantong and Labuk Bay
Gomantong is a marked boardwalk you can walk unaccompanied, but a guide turns a smelly cave into a functioning ecosystem with a centuries-old industry hanging from its ceiling — worth having if it's bundled, not worth a special hire. Labuk Bay needs no guide at all: the monkeys arrive on schedule and narrate themselves.