Boxcar
I can understand the company's thinking process in asking you to refuse offers of a higher level. They may have been taken to task in the past by ATC units further down the route who have been overloaded by additional aircraft apparently at non-approved levels.
However, I think a better way of dealing with the situation is, if offered a higher level, to reply that you have been level-capped and adding which sector or airspace you are planned to stay beneath (if indeed you know). This allows ATC to check if the relevant sectors can actually accept an additional aircraft. Remember, flow control is a relatively blunt instrument, and there may be tactical opportunities to improve levels and/or routes whilst you are actually en-route. Then, if the offer of a higher level is confirmed, you may take up the offer with a clear conscience and no risk of unwanted repercussions!!!
NN