It really depends on how long of a bar you're running. The main reason you see people asking this question is because their saws have adjustable oilers that can be adjusted to accommodate different bar lengths. On smaller saws, this really isn't as critical. If you're using 3/4 tank of bar oil for every tank of fuel, you're fine. I wouldn't go any less than that, but I if your oil pump isn't adjustable, I wouldn't go tearing into the saw in search of a problem either. If you're oiler is adjustable, and you prefer to run a bar that's on the long side for your saw (lets say you're running a 20" bar on a Stihl MS261), then you might want to tweak it slightly to give it just a little more oil. Honestly, on saws that are 60cc and under, you should be able to max the pump out without running out of oil before running out of gas, but if you go this route you will want to confirm this is true for your saw. On saws of that size, the oil tank is small enough that maxing the pump out won't make a big difference in your overall bar oil consumption, and you're better off using too much bar oil than too little. If it seems like you should be using enough oil based on consumption, but aren't able to see evidence of oil coming off the chain at the nose of the bar, then take your bar and chain off and clean the groove of the bar out, as well as the oil port on the bar, and the area under the clutch cover on the saw. When cleaning the bar, use something think to get down into the groove at the oil port. What ever you are using, you should be able to actually see the end of it through the oiling port. Chips will get wedged down in there and obstruct the oil flow.
If everything is clean, and you still can't notice any oil coming off the chain, run the saw WOT without the bar, chain, and clutch cover installed so you can see the oil dribbling out of the saw. If it is, everything on the saw is fine. At that point, I clean my bar off with some alcohol, and use a black sharpie to scribble on the bar around the oil port. Then I mount it on the saw, and barely snug the bar nuts. Then I wiggle the bar just a little and take it all back apart to see where the saw rubbed off the sharpie. This allows me to see how the oil port on my bar lines up with the oiling grove on the saw. I run Stihl saws, but on my bigger saws I normally run Oregon bars just because of cost. Oregon bars have tiny oiling ports and they don't usually line up very well with my oiling grove. I go through this process and then use a carbide bur in my dremel to open the bar's oiling port up in the correct direction.
When you get into bigger saws and longer bars, these adjustments can become more important. When I'm running a 28" bar on a 90cc saw, I'm normally only using about half a tank of bar oil and it's plenty (its a big tank). When I put a 42" bar on it, I max the oil pump out, and the oil runs dry a few minutes before the gas, and I'm barely getting enough oil on the bar.
