Leashes, Invasions, and Early Jungling
Leashes can be good or bad depending on not only the jungle's allies, but also the jungle themselves.
A leash should be quick, clean, efficient, and involve as little risk and damage to the jungle and no damage to their allies.
It is the preference of the jungle where they start, Red or Blue. Either option is viable for a number of reasons.
The jungle may also choose to invade, but an invasion requires the consent and cooperation of the team. The jungle may suggest an invasion or the team may suggest one, but without the consent of all involved, an invasion will not likely succeed.
Starting near the bottom lane of the map is generally considered the optimal start by the majority of the community. It involves two or more allies providing the jungle a leash as well as security against invasion. But that does not mean it is without its share of risks. The bot side is also where invasions commonly occur, involving the enemy adc, jungle, support, AND their mid. It is for these reasons that the ally mid should be present to provide friendly cover even if they have no intention of helping leash. It is unfortunate that most mid would rather loiter unproductively in their lane, thereby exposing their allies unnecessarily to potential danger.
A jungle should POLITELY request that their mid stand nearby until allied minions reach the first turret. This way, the allied mid can provide vision, potential defense against invade, and it will cost them no CS in the event of no invasion or provide the team with a potential first kill if their services are needed in combating an invasion.
Starting near the top lane of the map is generally performed by players whose champions may benefit more from the buff on that side early than they would from the other. Their early clears generally suffer as a result because the leash provided by only one to two allies means that the jungle will suffer more damage from their camp than they would if they started bot side. Invasions in top are not uncommon and may involve anywhere from 1 to 3 enemies and the protection provided to the jungle by one ally is extraordinarily insignificant compared to the protection provided by two or three.
Regardless of where a jungle starts, enemies will often ward both buffs so as to enable an early invade by the enemy jungle. They will determine where the jungle isn't so that the enemy jungle can safely steal and leave. It is often helpful for allies of the jungle to either do the same or to ward both sides of the jungle early depending on the intent of their jungle. The allied jungle should communicate if they plan on invading early or not and allies can then proceed to ward accordingly.
TLDR; Junglers should politely ask for help where it is needed. Allies should respond to those requests so as to make the game a more positive, enjoyable, and successful experience for the team. And everyone involved should always expect the worst case scenario in order to prepare for it and prevent it from happening.