It is almost always preferable to export the logs for the failed task.
In case you want to collect them manually or when te failure occurs before the task has started, then it depends on the situation. Worker log is always useful, server's and UI logs are usuful only occasionally. in case of V2V there are no additional logs.
In case of Windows P2V agent's log is useful; it resides on the source machine.
In case of Linux P2V helper's log is useful; it is located on the destination VM but for manual collection the VM need not be rebooted (it boots from ISO and has memory mapped disk during conversion)
Regards
Plamen