When I bought my dry suit, it was cold and there was no need for rash guards. Instead, the need was for a thick isolating layer (in my case the Kokatat dryliner). But as soon as the outside temperatur went above 10 degrees, it was time to ease a little of the shell under the dry suit. I tried my old fishing underwear, as well as some warm versions of dry suit innerlayer. I have made a general description of a kayak drysuit and its parts, this explains a little on what a drysuit is, and why the change of clothing beneath it becomes the important factor for adjusting your clothing to the outside climate.
When it had become yet a little warmer, somewhere around 15 degrees of water temperature, and then an air temperature around 20 degrees, I switched to pure rash guards under the dry suit. I could have switched earlier, but was careful due to the water temperature (which at that time was below 15 degrees). I preferred being a little warm in the air, to being way to cold if I would fall into the water. So if it was not for the water temperature, I would have switched to rash guards earlier. I currently believe, (based on both spring, summer and now autumn experience, where autumn is important since the water temperature will stay high but the air temperature is not high enough to quickly warm you after having been in the water) that a thin rash guard under the dry suit (I am now preferring NRS Microlite before others, no scientific good reasons) is enough from a water temperature of 15 degrees Celcius and up.
Also, when it is hot, when the sun does shine, like in summer when me and my wife first went out for a 7 kilometer long kayaking tour, and then went for an hour of kayak roll excercises, I used the rash guard without the dry suit. And the rash guard works well without the dry suit, it is a piece of clothing that does not feel warm, that does protect against the sun, and that also protects a little, only a little though, against wind and drops of water coming from paddling or spraying.
On trips where we do stops and launches from the beach so I risk the seat getting wet, I exchange the rash guard pants for thin neoprene pants or shorts. This allows me to sit dry even with a little water sipping into the kayak (except from launches, it will happen if we are playing and edging/bracing a lot, or if the waves are spraying the desk.
This is very much the beauty of the dry suit. It evens out temperature differences and makes selection of clothing easier. Increasing wind or loss of sun or spraying water or rain will not make a huge difference inside the dry suit. Thus it will not become especially much cooler inside the dry suit, and you are likely to do well with one and the same clothing a full day with all the variations that might come during a full day. Also, if the sun is too hot, or the wind dies out, a very good clothing is the rash guard you have under the dry suit. So I always carry a small dry bag to put the dry suit in, should I decide to take it off (likewise, if it is a longer trip and I start without the dry suit but I am unsure of how well it will work without the dry suit, then I carry the dry suit in a dry bag in the kayak).
July 2009 was the first time I tried the rash guard without the dry suit for a longer period in the water. I stayed exercising the Eskimo roll, and sometimes failing and staying in the water, for around an hour. We had unusually warm water, 22 degrees Celcius, so it did not test as much as it would normally do in 20 degrees.
Anyway, after 50 minutes spent to a large extent in the water, and to some extent sitting in the kayak, I was not feeling the slightest bit cold. I would have felt it cooler without the rash guard, so there is some warming done also by the rash guard itself.