I started last year in mid-August in SJPdP. I got a bed every time even if I arrived late in the afternoon in the albergue. So my personal opinion is that a thermarest is not necessary.
But just for the case I will take a sleeping pad (http://en.wikipedia.org/wiki/Image:Mattress.jpg) with me. It is not as comfortable as a thermarest but it has less weight. But I'm quite sure that this time it will be also totally unnecessary.
Last year from Le Puy to SDC I met many people who had thermarests-I never saw anbody use them. This year on the VDLP of the few people I met many had them, no body used them but did have problems when it rained because their poncho/backpack cover could not accommodate the thermarest. Remember the mantra "if in doubt leave it out"
Was the only thing I totally ditched in May!! Mailed a few items home...but the sleeping pad...Bye Bye...just left it in Redondela!! Omar is totally correct...when in doubt...leave it out cause finding the Correo to mail it home can be time consuming! Best of Luck, and Buen Camino, Karin