The answer partly depends on who your investors are and if there are any formal commitments in the investor docs, partnership agreements, etc. My experience has been that investors, specifically those who invest in start-ups/early stage companies really appreciate a monthly update, at least until the company stabilizes.
Burn rates and how cash is spent is so critical in the beginning as any errors in judgement are much harder to recover from. If you are using milestone based funding it means you are running lean and mistakes can be disastrous. Now if Uncle Mike sold his company and is investing part of his upside in your venture then you probably have more room for errors than most LOL.
I have not seen the CAP table change unless another capital raise is being done or some sort of exchange/transfer of shares is being done. If the company is not meeting it's revenue and the valuation cannot be supported in the next round, it becomes a down round, with the current investors equity being devalued. You should communicate the facts honestly and openly to investors. Assuming they are accredited investors ups and downs are expected and they knew this going in.
What should not happen is to take unnecessary risks that puts you in a place where you have to "spin" the "here's what happened" story out of fear of their reaction. If you commit to open and transparent communications with investors as part of your pitch then make sure you do so, 100%, no exceptions. You will maintain your integrity and their trust, which are far more valuable than a few unpleasant conversations.