You can get away without having a meter. However, we are always searching for optimum conditions, reservoir control being one of the most important factors. A ph and EC/ppm meter will cut the guesswork out of your nutrient solution. Not only will you have complete control in reading ph, which is VERY important, but know when your plants are feeding with a drop in EC/ppm.
When buying a meter, keep the probe wet at all times with plain tap water. Handle your meters carefully because they are delicate pieces of equipment. Clean them regularly to avoid bacteria buildup (you can do this by using plain dish soap, alcohol, or hydrogen peroxide).
Hanna, Milwaukee, and Nutra drip all make awesome meters. I own a Nurta Drip tri meter and a Milwaukee tri meter. Both can be had for around $160.00 and offer great quality and complete control.
In dwc, water temp is important to keep bacteria growth to a minimum. If you are running hydro organic ferts, a good beneficial bacteria is essential. If your running chemical ferts, h202 or a product like Dutch Master Zone is a good idea to keep the bacteria from causing suffocation and root rot. The amount of DO in water with a temperature of 80 vs. 68 is only 1 ppm, so your not missing a lot of DO in a warmer res. A warmer reservoir will result in faster growth both of plant and the root zone but can cause an outbreak of bacteria.
My advice would be, if your running organic with beneficial bacteria, keep the res 70 and under. If its warmer, run chemical based ferts.