Julian,
mandatory?
Technically, yes. Most have upgraded. You are highly encouraged to upgrade ASAP as this release takes up less resources too. The exchanges Bittrex and Bleutrade have also upgraded. Cryptsy is unfortunately running 1.7.
In this change in the 1.8 release:
commit 9dd0268580d3c73ccc77ae512d7203bb9b1fc6ff
Date: Sun Sep 13 22:55:21 2015 -1000
Change Proof of Stake rewards to change based on nTime as opposed to block count.
The next reward change kicks in at exactly 2 years since launch:
if (pindexBest->nTime >= 1472688000) // Thu, 01 Sep 2016 00:00:00 UTC
Previously it kicked in based on block count:
if (nHeight >= YEARLY_BLOCKCOUNT * 2)
Where YEARLY_BLOCKCOUNT was set to:
static const int64_t YEARLY_BLOCKCOUNT = 525600; // 60 * 24 * 365
So you definitely MUST upgrade prior to block 1,051,200. Currently at block 606,987.
The next update will most likely be a hard fork so that will be a mandatory update but I'll give ample time to upgrade that.