League of Legends (LoL) Question: Backing and the Wave

It should be exceedingly rare that you want to back for an item at full health and mana. Recalling for an item gives up pressure on the map while you're gone, so I wouldn't recommend doing it unless you're low on health, low on mana or have a very important item spike to pick up. Don't just recall because you can afford a Blasting Wand. If you have lane control, you should look to push in and either punish your lane opponent or roam and ward / gank other lanes.

If you really do need to recall to buy, use your mana to push your lane quickly to the enemy tower and then recall to ensure you don't miss gold/XP while you're gone. The wave should also bounce and start pushing to you.

To be honest, if you often find yourself in the situation where you have a ton of gold with full health and mana, you probably need to be playing more aggressive.

Universally, one of the best times to back is when your enemy laner backs. When this happens, kill the minion wave as fast as possible to get your wave under tower. This will cause the enemy to lose farm. Then you can back. When you return to lane, the enemy will have centered the lane for you. Then you can repeat this process. If you have high health when you push the wave, that is when you damage tower. Unless the enemy is dead or has backed themselves, you do not want to back. If you are low health, however, you usually cannot avoid backing. DO NOT STAY IN YOUR LANE ON 3HP JUST TO AVOID LOSING A FEW FARM.

The way waves work:
(sides mean blue or red)
1) first side push, so the waves will go towards the second side without champions in the lane.
2) who will push? Even minions wave engage on the first side will push to wards the second side as the reinforcement wave will come first.
3) more information on every SRO video.

Now your question is when to back.

The way to make enemy lose gold is by getting your team minions under tower so he won't get gold or EXP (if he not in the lane).

While the wave is under tower you can roam, back, invade jungle.

You should back if you will achive a good spike with an item or if you sure your opponent just used his gold and you don't want to be behind (gold that have not been used worth nothing).

You should roam if you see the lanes neer you are gank able or the jungler making his way to there to gank (so it will be 2 man gank).

You should invade jungle if you have enough vision and know where anyone of the enemy team on the map.

Remember to take teleport to mind as you want to save it for ganks or backing from ganks and not just from the nexus to the lane. And pay attention to your opponent teleport.