. . . I know that the same trunk member is used for return traffic to the source device.
That depends on if the hashing algorithm being used hashes the same attribute. For example, if outbound traffic to a destination used src-mac, and return traffic used dest-mac, then the same link would be used because the same attribute, i.e. the same MAC, was used used in both directions.
However, will load-balancing with two port-channel members mean that 5 of the source devices always choose member trunk 1 and the other 5 member trunk 2?
No, it doesn't. Again, the link used would depend on which link the hash algorithm determined based on the attribute(s) being examined. The algorithm will, though, given the same attribute(s) chose the same link, assuming the number of links is the same.
Ideally, the hash algorithm being used will equally distribute traffic across all the links in the Etherchannel, but this isn't guaranteed. So, for your 10 devices, the odds of them being equally split across both links might be about the same as all 10 devices using the same link. More likely, there will be some imbalance such as 6 or 7 devices using one link, and the remaining devices the other.
BTW, if just using a single attribute like MAC, from a gateway to multiple devices, you would normally use dest-mac for traffic from gateway to the other hosts, and for the other direction, from hosts to gateway, use src-mac. If the device supports it, you might also use, for such, src-dest-mac for both directions. What you want is some attribute that differs between flows.
