With tapping I mean tapping downwards, to settle the grounds and not tapping the side of the portafilter.
I ask, because I use a LM ridged double basket, which I overdose with (around) 19g and a 58mm tamper. (I tried a 57mm tamper as well and although that isn't hindered by the ridge, I don't like the fact that the coffee creeps up between the basket wall and the edges of the tamper base). I overdose because I like the stronger shot and because of the ridge in the basket.
When I tap the pf downwards (lightly) it is possible to get more coffee in the basket but then it has to be done before levelling of course. But when I do so, should I repeat the tapping after the levelling and before tamping ?
AND, would this be a good idea with the LM double baskets:
tamp lightly first with this tamper
http://espressocare.com/Qstore/Qstore.c...onvex+58mm
which I bought a few years ago and which is 57mm at the bottom of the base and 58mm at the top.
And finish the tamp with my regular 58mm tamper, which is a Concept-Art Technic and adjusted to around 15lbs ?
Ton

After watching an Orphan Espresso video I now tap downwards, which generally levels the grinds, then finish leveling with a flat tamper with probably less than 5 lbs of pressure.

I use one of the Orphan Espresso dosing funnels when the mound of grinds has a chance of overflowing the basket. I dose with the basket out of the PF handle, shake it a bit (side-to-side) to even it off, snap it into the bottomless PF handle (which settles the grinds), remove the funnel, tamp moderately (once), lock (carefully) and go.

While I have my good pours and my occasional bizarre pours, I don't see any obvious or consistent "donut" or one-sided extractions.

depends on your end goal. If you are simply trying to stuff more coffee in the basket, fill it about 2/3 full, level out the mound, give it 3 or 4 thumps on a tamping mat then finish filling. I have been thumping my portafilter for probably 3 years now. I do it to settle/distribute the grounds just before tamping to help level out the dosing.

How, when and if you do "tapping" or "thumping" depends on what you are trying to accomplish. As variously recommended in this thread, the three typical purposes are to:

A. Control the amount of coffee in the dose, or
B. Level the dose in the filter basket, or
C. Knock residual coffee from the filter basket side-walls

Regarding (A), different grinders and different coffees can have remarkably different "fluffiness," but once you get used to a combination, you can maintain very tight control of the dose by using a tap+level sequence. The benefit here is that by tapping a "fluffy" dose that is mounded above the rim of the filter basket, you can adjust to the right dose as it compacts downward, then level off that correct dose (with a some kind of straight edge) evenly at rim height - which is much easier and more accurate than trying to level a dose under rim level. This technique seems to work better if, as Jeff mentions above, you dose and level with the filter basket outside the PF. This, because you can tap lightly if needed and directly upon bottom of the filter basket this way. If you choose this method, my experience is that it's better to tap evenly around the bottom edges/corners of the basket, since tapping the center of the basket bottom is more likely to unevenly compact the center of the dose, leading to donut pours. Tapping at the edge also seems to help prevent edge channeling by giving the sides a little more compaction. "Thumping" downward with the basket in the PF can get you some of this benefit, but you won't have the same finesse or control given the mass of the PF dampening the vibration and your control.

Especially if you keep the basket inside the PF, (B) is commonly used for leveling, and in practice it seems to work pretty well, probably since the grinds seek the path of least resistance given gravity, and so the theoretical effect of uneven density doesn't seem to play out much in practice.

Regarding (C), that technique seems to be pretty much discredited, since it would occur after the tamp and opens up the opportunity for breaking the edge seal resulting in side channeling. Most of the experienced writers on this site have advised to either (i) ignore these residual grounds, (ii) scrape them downward to the edge with your tamper (gently enough not to disturb the puck, and this may actually protect against edge channeling) or (iii) get a tamper that's a very, very close fit to your baskets, which prevents most of those rogue grinds to begin with. There's been one more suggestion written lately, which is to withdraw the tamper suddenly enough to create suction that removes any powder clinging to the edge. It seems the jury is still out on that one, with concern voiced by some that this has the potential to dislodge the puck slightly from the basket, even if not visibly.