Should I start timer immediately after adding pasta to boiling water?

You should start it as soon as you dump the pasta in the pot. I personally also turn off the heat at the same time, as you don't need your water to be boiling to cook pasta. Anything over 180 degrees Fahrenheit will do the trick. You could stick a thermometer in the water to make sure it's still hot enough, but I doubt it's possible to drop the temp by over 30 degrees unless your pasta is very cold. If you wait to start your timer and the water is still over 180 degrees, your pasta will be overcooked.

This is not true for ravioli. The pasta will get done at 180 degrees, but the filling will not. Using more water will cause the temp to drop less when you dump in the ravioli, and you should definitely keep the heat up so it starts simmering again as quickly as possible. Hopefully that's not long enough to make a difference one way or the other for your timer, as the pasta will become overcooked if it sits in hot water for a few extra minutes. I'd still start the timer when you dump the ravioli in and then check it when the timer goes off, but expect that it might take another minute to finish cooking.

The problem arises from you not using enough water. Traditionally, pasta is cooked in a very large pot with lots of water, I believe I have seen directions for a minimum ratio of 1:10 (a liter of water per 100g pasta). If you do that, the water won't stop boiling and the timing will be clear.

Despite what some purists say, this is not the only way to successfully cook pasta. You can easily use other methods, including such with less water or a temperature different than a full boil. But if you do so, you can no longer rely on the time printed on the box, and should cook the pasta by doneness, not by a timer.

I don't think it's a silly question, I have wondered it myself. I find I have results closer to the time indicated on the box if I wait until the water is boiling again before starting the timer.

I would make a distinction between dried and fresh ravioli since they need to be cooked in a different way.

Dried ravioli: I would bring the water (ideally quite a large amount) to a rolling boil and salt it. Then add the ravioli and turn the burner down to half power such that the water remains hot (ca 90°C 190°F) but not boiling. In my experience using boiling water can cause the ravioli to rupture. Let the ravioli cook for the duration mentioned on the packet - start tasting one around 30s before package time is over to check if it's done.

Fresh Ravioli: Bring the water to a boil, salt it and turn off the burner before adding the ravioli. The problem with boiling water bursting the ravioli is more pronounced on fresh ravioli as they are more delicate. Add the ravioli to the water and wait for them to float - should take about 40s to 2min depending on how thick the dough is. The filling mostly just needs to be warmed up rather than cooked as it's made from eg. ricotta cheese or already stewed meat.

In general if you are worried about the timer I would recommend to rely on tasting and the look/feel of the pasta to discern if it's ready. I use the time on the box as more of a guideline since I find the time given on packets is often inaccurate anyway.