This is a good question - it touches on some important issues when doing concurrent design. As has already been stated, the answer to your specific question is, according to the current implementation, FIFO based. It's unlikely ever to be different, except perhaps if the implementers decided, say, a LIFO was better for some reason.
There is no guarantee, though. So you should avoid creating code that relies on a particular implementation.
The broader question concerns non-determinism, fairness and starvation.
Perhaps surprisingly, non-determinism in a CSP-based system does not come from things happening in parallel. It is possible because of concurrency, but not because of concurrency. Instead, non-determinism arises when a choice is made. In the formal algebra of CSP, this is modelled mathematically. Fortunately, you don't need to know the maths to be able to use Go. But formally, two goroutines code execute in parallel and the outcome could still be deterministic, provided all the choices are eliminated.
Go allows choices that introduce non-determinism explicitly via select and implicitly via ends of channels being shared between goroutines. If you have point-to-point (one reader, one writer) channels, the second kind does not arise. So if it's important in a particular situation, you have a design choice you can make.
Fairness and starvation are typically opposite sides of the same coin. Starvation is one of those dynamic problems (along with deadlock, livelock and race conditions) that result perhaps in poor performance, more likely in wrong behaviour. These dynamic problems are un-testable (more on this) and need some level analysis to solve. Clearly, if part of a system is unresponsive because it is starved of access to certain resources, then there is a need for greater fairness in governing those resources.
Shared access to channel ends may well provide a degree of fairness because of the current FIFO behaviour and this may appear sufficient. But if you want it guaranteed (regardless of implementation uncertainties), it is possible instead to use a select and a bundle of point-to-point channels in an array. Fair indexing is easy to achieve by always preferring them in an order that puts the last-selected at the bottom of the pile. This solution can guarantee fairness, but probably with a small performance penalty.
(aside: see "Wot No Chickens" for a somewhat-amusing discovery made by researchers in Canterbury, UK concerning a fairness flaw in the Java Virtual Machine - which has never been rectified!)
