I have always felt that with any animal if you can mimic its natural environment it will be happy.In the relatively small area of their enclosures there is little margin for error with tarantulas etc.
My first tarantula was about 11 years ago and I dutifully kept it warm and dry but it never thrived.
I then went through a high humidity phase and now am back at keeping adults at a low humidity level. GBB's definitely want it dry but Brazil anything types enjoy a much higher humidity.My slings are kept with condensation on the sides.I rarely lose a T and the average temp is 28,lowest 25.
Some authors say there should be no condensation on the enclosure sides and if there is humidity is too high.
Its hard to imagine the desert species enjoying a higher humidity but evidently they do as their burrows are deep and cool.
It will be interesting to hear from you guys what you feel about ultimate keeping conditions.

I believe to give the tarantula a choice, and always keep a portion of the substrate moist.
If you have a plant in the enclosure it will always be that spot otherwise keep one of the corners moist. Some species will always go and sit on this wet patch during the heat of the day, others will avoid it. This also helps in keeping the humidity up.
Humidity is important to keep the tarantula hydrated.
Condensation on the sides of the enclosure will only form if the temperature of the inside of the enclosure and the outside differs. With good ventilation this will evaporate quickly.

i agree with hendrijs,i always keep a part of the enclosure moist and found that the t's move to the part of the enclosure they want at the specific time during the day my b.vegans sits close or in the burrow where its dry but found that during nigtht time it prefer to go to the more moisture rich end of the enclosure all depends from t to t and species