In the situation you describe, I found when I was doing my navs (in another era), that if I drew the ATO's attention to my 'problem' and 'discussed' it at the time, then that helped.
And when I was instructing, I did appreciate the student letting me know what he was thinking, so at least I knew he was considering something...
He may give you the advice you need, even in the form of a question - as long as you make it clear that you are considering this, or that, to avoid 'breaking' the VMC rules, and you are prepared to make the decisions.
It is a PPL test, and although you do have to display your skills and make your decisions, it ain't a CPL or higher test, and your experience level is not supposed / expected to be all-encompassing at this stage....
The rest is how 'comfortable' you feel - as 'Aerocat' so aptly says, fly horizontally far enough that you feel comfortable, and make some remark if necessary to that effect, and during any training you have prior to the test, see if you can keep the cloud base 'well above' the horizon.
That is a guide to your vert. separation- if your 'circle of vision' to the far horizon is getting smaller due cloud above, then you are far too close and need to descend accordingly.
I guess you could always try to fly at least 1,000ft below the forecast cloud base, and see how that turns out.
In any case, try to enjoy as much as you can, talk to the guy if that is 'within the current rules'....
Good Luck
