Developers are not composed on one ingredient but several. Depending specifically what you are using, some developing agents might be more
affected than others in nonstandard temperatures. This could affect things like final image color. In other words, "completion" might come sooner with respect to one ingredient than another, if you even want to go that far. This has more to do with fine-tuning your results. And it
can be an issue if you are trying to replicate an effect you got during a significantly different time of year. The most important thing is consistency, so you can realistically predict your results. You can always use a tempered water jacket too - a larger tray around your actual
developing tray. You also have to be aware of perspiration on your fingers affecting materials - it's a chemical agent too! I'm lucky to have
one of those old Zone VI compensating developing timers - it works remarkably well for modest changes in fluid temp.