I recommend 35BB,check out the table after 20-30 hands if you see players making obvious mistakes or the table has a wild spree in their play then buy-in for the maximum. Though if you notice all the players are tight and aggressive and they are feeling you out correctly then the correct thing to do is leave. Rock's have the least volatility and least amount of leaks.
Get comfortable buying in with full stacks/playing deepstack poker. It will greatly benefit you in the long run. Top tier players are very comfortable in these situations and can usually spot players that are not.
Maximum buyout obvious.The deeper the stacks the better the game.You cab really relax and give your best.Also you have enough money to experiments with the theories that you read from the books or to change your type of plays numerous ways to make your opponents very hard to read you.
Always buy in for the max if you can afford it. There should be players at that table with 500$ + in front of them and if you truly want to compete with them you have to buy in for the max.
That being said if you have a smaller stack they are more likely to bully you with bluffs due to you seeming to be more of a nit because you only bought in for 150$ or so.
To me this all comes down to your "edge" if you think you are the best player at the table buy in full - if you dont think you have an edge you should table select better. Or if you think you have an edge with 20bbs, 35bbs, 40bbs buy in for that amount.
in my opinion at a cash game no matter what stakes, its always good to observe the table to see how your possible opponents may be playing. if the table has a lot of loose short stacks i'd suggest bringing a max buy in to cover an all in bet if your opponents are sitting with 20BB-50BBs and wait for good spots, other whise 50-100 big blinds is usually fine, i normally buy in with 80BBs.
Edit, I usually buy in for 80BBs because i find that i tend to get more action when i don't buy in for max BBs because i tend to find people call off my big river bets etc when i'm shorter stacked then them, or are more likely too.
Bankroll is very healthy, because talk about bankroll, buy in, certain to have a cash management is dedicated, for many you need to have 100 buy in, another 1,000 buy in, this will be relative
Always play max buy in, ALWAYS! Just know when you’re beat and lose the minimum.....I know, sounds easy right? Lol. I look at it like this. If there are two tournMents to choose between playing , and your bankroll allows you to comfortably play either of your choice, why play and win the smaller buy in when you could win more $ in the higher buy in? Look at cash games the same way!