The first question you must ask is ‘What is the market for similar units today?’
Market rates can change quite a bit in both directions.
If an area is suddenly overbuilt or there is the loss of a major employer, rents can go down. In those cases even keeping the current rent may cause you to lose a great tenant.
If the rents have gone up a lot, then you may be leaving too much money on the table.
You can look at a bunch of local ads and Craigslist to get an idea. This can be a lot of work. I cheat and use rentometer.com (they have a free level, but we do enough to make the pro version worth while) Then go and look at their comps to see where you should be as all three bedrooms in Anytown, USA are not created equal.
For a great tenant we do not raise to market, but will often raise a bit. There is a cost to turning a unit in prepping, advertising and lost rent.
If you are not planning on selling the place, getting the last 5% of rent may be a fools errand. On the other hand if you are selling a place, 5% of additional rent can be thousdands of dollars of added value.