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. ;)

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=

Dear Moonglum, Unfortunate many or maybe most LPs are not perfectly flat.
Using Reso-Mat for a convex LP there are only the 3 inner points contacting record. The grooved area, actually the whole record is spinning in air.
The other side is concave and thus there are 6 outer points in contact. This case is sorted out with light clamping and thus all points are in contact.
Sonically both cases may very well benefit from light clamping. On the other hand too heavy clamping ruins everything in achieved sound improvement. I use clamp only for flattening concave LPs.
In my system Reso-Mat, and particularly tall Reso-Mat improves sound quality and quantity in all frequencies, and with all records. After 2 years I have not experienced sibilance nor other issues caused by Reso-Mat. I have one "top" MM that suffers from annoying sibilance, even with new stylus (a false sample or a false "god").
In my system lifting record off the platter seriously improves the sound. Music just flows nicely. For me there´s no turning back to conventionl mats.
I must point out that I still use the original dense and soft mat (GROOVE ISOLATOR) under Reso-Mat because it eliminates ringing of the metal platter.

Dear Moonglum, Quite rightly so :-)

I´m planning to buy a 1 kg mat under Reso-Mat, not to improve sonics but add mass on the maglev bearing.
And oh yes, for fun too... : )

Moonglum & Dave, Very well I will try the 233 g brass clamp again but if I remember correctly it doesn´t improve things because it may be too light with Reso-Mat.