It really depends on your rank and the bot lane.
I generally calculate the potential gains from a gank or tp play bot vs what I KNOW I will lose via leaving my lane and having to walk back there afterwards. Bear in mind a KEY part of this calculation is that... if bot lane doesn't decide to intentionally feed just because they fell a bit behind, they WILL scale up just fine and have relevant damage for teamfights and waveclear - something that cannot honestly be said about any other lane right now. Thus, I usually err on the side of caution and do NOT tp gank or lane gank bot. Once in awhile if the mere threat of a tp can stop a 3v2 or 4v2 tower dive onto my bot lane, I'll do that. Otherwise it's generally best - as boxerpete says - to leave bot lane alone because chances are they will end up feeding even if your tp play or gank puts them far ahead. That said, I did do a tp play yesterday very early in the match which I was certain would fail; however, it ended up saving my bot duo, netting us more kills than it should have, and bot actually played intelligently and snowballed off it. Sure there were a few setbacks, but unlike most games, they didn't decide to run it down lane forcing bad fights and ruining our chances of winning; they took smart fights, avoided bad fights, and won their lane. They even managed to... make rotational plays to help teammates snowball afterwards - something I almost never see from bot lanes in silver or gold.
When to gank for bot:
your lead in your lane is massive - not tenuous
when your lane opponent is already there and you cannot get more by splitting
when you've taken the outer tower in your lane and shoved a giant wave all the way in to your opponent's tier 2 tower
when a dragon is up or about to spawn
when you can stop enemy getting first tower gold and pick it up yourself via ganking bot (sometimes worth)
bot lane failures when given a lead via ganks - these are all too common in silver and gold:
After getting a lead, they don't punish their opponents. They trade farm passively instead of pushing in and looking to rotate mid for tower damage or swapping lanes with toplaner.
After getting a lead, they don't focus on killing first tower bot - they keep greeding and taking insane risks for kills. Eventually they end up being preyed upon for their risky tendencies, they feed away multiple kills, and they lose towers when they're dead - putting the other team far ahead.
Bot lane is their universe. They only push bot, only go for towers bot. Why go for a risky tier 2 tower bot when you could do the much safer and smarter option of... pushing in and rotating for dragon or mid tower? The only reason you need to go bot again is to waveclear when it's pushed close to your tower.
No teamwork and no warding. They just run wherever they want, take whatever jg or lane farm they want and refuse to work with their team. They're fed, so they're gonna 2v5 carry the game and no you cannot get any farm. This leads to them repeatedly taking 2v5 and 3v5 fights they cannnot win, throwing away their lead, costing objectives and towers, and eventually the other team has a huge lead even if towers and kills are even due to shutdown gold, objectives like baron/drag, etc. Meanwhile, their teammates are starved for gold and xp and have no desire to work with people who screw over their teammates and flame them just because they're not hella fed. Never mind the fact that top, jg, and/or mid are generally the REASON bot is fed.
*if bot is feeding - don't gank for them jg. If bot takes stupid fights because they won't listen to team or be logical, don't gank for them. Let them feed - it's better for you to gank for a neutral or winning lane or counter-gank somewhere where you COULD be winning if even in numbers than to try to prop up an intentionally feeding lane. Go elsewhere, get a more intelligent teammate (and yourself) fed, and make plays where you are strong. Either bot will realize they need to stop inting and you can use your lead from ganking elsewhere to stabilize them THEN or... they'll continue inting and you at least have a lead elsewhere which can potentially be used to neutralize the fed enemy bot lane or just snowball harder/faster than it.