turntable mats: should I use one?

No way. Don´t waste your time & money on mats whatever they are. They change the sound, sometimes may improve it in some extent but not improve it as whole.
Buy a Reso-Mat instead. Vinyl sits on acetate spikes allowing resonances vanish into air and not bouching back from platter´s surface to cartridge smearing the original sound from your stylus. It just works. ;)

Harold...I just ordered the Resomat to add to my collection. I'd never heard of it before but cheers for the heads up m8 :)
I'll be comparing it to my existing (undamped) mat.
If I like it, it could become permanent. :)

The idea of parking the LP on a bed of nails is a wee bit unnerving but the only thing that concerns me is the proximity of my platter's label cutout. How close are those cones to the label area?

Been using Herbie's for years. Now I use the Moomat.

The inner cones are 0.5 cm from the label area. Actually all the cones are under the run-in & run-out areas.
The reduced size (for Technics SL-1200 etc.) has the outer cones under the outer groove area:
http://www.ebay.co.uk/itm/RESO-MAT-TURNTABLE-PLATTER-MAT-FOR-TECHNICS-SL1200-GARRARD-401-/181111989567?pt=UK_AudioTVElectronics_HomeAudioHiFi_Turntables&hash=

Sorry to report that the outcome of the Resomat comparison was disappointing. 

Theoretically my thick, heavy composite, non-resonant platter was a match made in heaven and the ideal platform for the Resomat to show off its wares so there was a great deal of excitement and expectation on approach.
Although the signal was clean enough I could summarise the general character of the Resomat as lightweight and insubstantial. Sibilants were slightly exaggerated although not excessively so. (You could get used to it).
I couldn’t resist looking underneath to see how many contact points were supporting the LP and found that only the innermost 3 on the LP in question were actually working.
This may lend credence to Ynwoan’s alternate design which uses only those 3 points but lightly clamps the LP from above - meaning that the playing area is unsupported. That would probably help to control sibilance and add more meat to the bones?

The Resomat was claimed to create a wider soundstage but perversely it was actually the GS Ringmat that generated the sense of a wider soundstage allied to a great deal more presence and bandwidth on all instruments – even those “outside the stage”. Sibilance with the Ringmat was much better managed and balanced. Its resolving power and sense of realism on percussives etc is first class. Basically, you could be unaware of many of the sounds that the Resomat was trying to create simply because the frequency balance was off-par.
Consequently it was an easy win for the GS Ringmat. Bear in mind that the GS Ringmat, at £85, is 3X as expensive as the Resomat but nowhere near the $500 needed to secure a Zanden mat, so if the Ringmat is defined as a good buy then the Resomat is a positive bargain(!)

Needless to say there are plenty of Users who are more than happy with the Resomat (although it’s unlikely that many of them have tried the mat or the hybrid combination that I’m using) and I wish them all the best because the Resomat is still a good mat, simply not the mat for me.
Despite the failed comparison I will be retaining it. 
On a positive note I found the Resomat a better listen than the Achromat.
….and you’ll never catch Lyme disease from it….. ;)