Would you rather receive a 30 minute slot delay to ensure that you get your desired cruise level every day? ATC do the best they can, especially in Europe where there are more planes trying to share the skies than separation normally allows. Besides, unless the ATC are holding you down by an order of 10,000 ft or more, the contingency fuel in your flight plan should more than account for the different altitude. If it doesn't, then it seems to me that an ASR to your company would be a good start at correcting the overly tight fuel planning you're being asked to accept.
BTW, if you're flying in Europe in today's environment, it's pretty hard to say that a lower than planned altitude qualifies as "completely unforecasted".