I always keep a box of Le Pros, Elk Master, Water Buffalo, and G12 break tips. Those I always have on hand because they're something everyone is familiar with, and cheap enough to keep bulk on hand.
I usually try to keep a few Kamui Clears, KO Brothers, Ultra Skins, Tiger Sniper on hand because they're popular for various reasons, but I don't always keep em and I don't bulk stock them due to price. I may honestly never install 100 kamui clears in my life, because I try not to do too many tip replacements. I don't honestly make money off tip replacements the way I currently do them. I mainly offer it as a fast turn around service for friends and people in my leagues. I charge $15 for a tip replacement if you supply your own, or $15 + cost of tip if I supply tip. $15 isn't buying dinner now a days, and I am not doing 20 tips a day, so it is what it is, but I ain't spending thousands on tips to have them in stock.
I've considered bringing my mini-lathe to tournaments and shows and doing replacements, but I'd have to charge extortionate prices to make it worth my time, and I don't roll that way, so I am just not going to do that.
So, long winded story short, stock the affordable tried and true tips, and keep a few on hand of the popular fad tips, but don't bulk em because fads come and go. The tips everyone went nuts for 10 years ago is different than today.