If you ask to report X team mate for a legitimate reason (AFK) such as they left the game 4 minutes in and never came back, would you risk your own account being banned? Assuming nothing else was said and no one was trolling to make that team mate AFK.
Yes, and no. A single instance may not necessarily result in validating a report, but continuous rallying for reports can and will.
Would the better choice just to report the AFK after the game and hope your team mate and everyone else does as well without you asking?
Yeah - as PH45 said, the IFS only requires one report (more literally, any number of reports more than zero) to trigger a review, so getting more people to report a single person has no greater effect than if you alone reported them. Ultimately, there's no benefit to asking other people to report someone; best to just do it yourself and avoid risking a punishment down the line.