Well, during laning phase, roam only when you know it is absolutely safe to do so. You don't want to randomly throw your ADC into a 1v2 situation that end up turning into the enemy's favor; I have won many games simply because the enemy support roamed at the wrong times, therefore giving my botlane a huge advantage by easily 1v2 the enemy ADC. If enemy bot is MIA, and your ADC just recalled, that's ok to roam.
Usually, if you are ahead, go somewhere else and do something there. It can be push and try to take down towers, take dragon, go babysit someone that needs help, you decide. If you are absolutely clueless, go mid and just try to get the tower.
If you are not ahead and/or is behind, it depends on where the enemy laners are:
- If they are going mid, go mid with them, while leave your ADC to farm so he can catch up.
- If they are staying bot, you have 2 choices: go mid, forget about botlane and just push/snowball mid instead, or stay bot with your ADC and hold it until you receive proper assistance (though in this current snowbally meta, not recommended since if you're behind, everyone else is behind).
Just a lot of "up to you to decide", really.