I think the answer is very individual specific. What I mean by that is none of us have the same flexibility in our blood vessels that is necessary to counteract against blood pressure, cardiovascular efficiency and tolerance to high-levels of blood pressure. Some people may handle it, some may not. Some may even be tolerant to very high-level of blood pressure. I think before ever attempting to Valsalva, one should prepare his body such that side effects associated with high blood pressure won't harm the individual in serious ways. If you have weaknesses in some part of your vessels, such move can end up with internal micro bleeding(s) without you ever noticing them. I can never tolerate upside-down shoulder press for example. It makes me feel like as if my brain is gonna explode. But some people seems to be very fine with it.
It is also important to keep intra-abdominal pressure high as well. I think you should start by keeping your core tight and exhaling through a narrow opening like pursed lips. I wouldn't start with holding breaths right away. Give your body some time to adjust to increased levels of pressure. Or start with light weights if you'd like to feel how holding breath affects your body.