You've asked two questions. About whether it is important or not to be consistent in speed when transmitting, I would say that the important thing is to get the information across at a speed comfortable to the receiving operator. If the operator is not as good at copying numbers, then slowing down the RST is perfectly appropriate. If the receiving operator is copying the address for the QSL card, then slowing down is appropriate. It's usually better to get the information across the first time than to send it fast, and then have to repeat. So I would say that yes, it is appropriate to slow down parts of the QSO to suit the receiving operator. Like @k6ekb says, it can also be appropriate to speed up the routine parts of the QSO, such as the third or fourth identification of the two stations before an "over".
To illustrate one thing that the sending operator shouldn't do, there was an incident when I was a new operator with US "novice" privileges. I sent my shaky CQ with a straight key at about 6 wpm, and I was answered by a European station, my first DX! In my excitement, I probably started sending a little too fast: I was easily capable of sending faster than I could receive. The other station, which had started at a speed that was comfortable to me, began to send faster and faster, despite my "QRS PSE". Soon he was sending about 15 wpm, far faster than I could copy. In a panic, I "pulled the big switch", and switched off the transceiver, and abandoned the shack!
Regarding the second question about whether it is better to slow down or repeat when conditions deteriorate, I would say that the answer depends on the conditions, the situation, and the skill of the receiving operator. I most often experience deteriorating conditions, usually interference and fading, when contesting. In a contest situation, it's almost always better to repeat the information at the "normal" speed, which is usually about 30 wpm, rather than to send it slowly. In a contest with QRM and QSB, it seems as though there is clear reception for periods of about one to three seconds; at 30 wpm, that's hopefully enough to copy the entire "payload" (crucial part) of the exchange. When the other operator sends slowly, I'll often miss one or two crucial characters, and have to ask for several repeats. So in a contest, if conditions are really bad then repeat the exchange two or three times at normal speed, but only if you're asked for a repeat. Many contesters have amazingly good ears, and can copy accurately despite conditions that would baffle an average operator. I hesitate to recommend the same answer for all circumstances, but in general I'd personally lean towards repeating the information at normal speed in bad conditions rather than sending slowly.