I happen to find the rough nail files just perfect for light tip maintanence (100 to 150 grit ones).
As you slowly turn the shaft begin near the edge and with light pressure gently s****e the file along the tip tangential to the already established curvature. Watch the tip as the file removes the chalk and you can see where the tip was dimpled from hard hits. Work these down until the whole tip is even in the rotational direction. Most of the deep dimples are found in the center from hard hits.
If you hold one end of the nail file on a pack of matches, and the pack of matches on the shaft of the cue. The angle of the pack of matches allows you to trim down any overhang of the tip beyond the ferrule. Just slide the nail file with the pack of matches up and down the shaft while slowly turning the shaft.
When I had a LePro tip, I had to scuff it about every other week for optimal chalk holding. My Everest is better about not needing so much maintanence. I will soon be trying out a Talisman hard--as the Everest is a little soft for my playing.
Also note: chalk in a humid climate is only good for a few hours--to a few days. You can restore the chalk at 220 dF in an oven for 20 minutes. If the cahlk will not squeek on the tip while being applied, it mayhave become too water saturated to do its job well. I buy chalk by the gross and leave it at the bar when I'm done playing.
It is time to replace a tip, A) when you lose confidence in it, B) when you have groomed it enough times that the edge is thinner than 1/16 inch (1.5mm), C) when you want to try something new.
Tip hardness*: You might think that a soft tip will grip the cue ball better during the moment of impact. The reality is that it is not the tip doing the grabbing, but the chalk crystals. The job of the tip is to be stiff enough to allow the chalk to be placed onto the surface and hold it until the moment of impact. A soft tip will allow the chalk crystals to embed in to the leather and not grip the cue as well. Thus, the tip needs a certain minimum hardness. This minimum is dependent upon how hard the player hits the average cue stroke. In addition the chalk need a particular amount of pressure at the moment of contact to bite into both surfaces. Medium tips should be about as soft as anyone considers.
The harder the tip, the less curvature maintanece but maybe more surface roughtness maintanence. If you have a tip on your stick that requires mushroom grooming more than once a month--it is simply too soft.
Really hard tips are required to perform above-the-equator jumps, and break cues should also have vary hard tips if you break with a great deal of energy.
I have a friend who plays with a hard tip when playing 9-ball, and a medium tip when playing (pannama) 8-ball. This is due to the difference in finnesse between the games.
Finally, there is some trade-off between tip hardness and ferrule hardness. Ferrule hardness has been comming down over the last 2 decades (excepting for jump and break cues). Ferrule hardness determins how the edge of the tip responds when applying lots of english, follow or draw, and does little when stroking in the middle.
(*) there used to be long hard rubber tips that would hold chalk like the dickens and still play as if they were medium soft. The leather ones we can use these days do not have these properties. If you have one of these old tips--take care of it.