Rescuing a dog is a great thing to do, but do bare in mind that many are there as a result of a behavioural problem of some sort. Some rescues are not for the faint hearted, and ownership should be entered into with eyes wide open taking particular care not to jump in hastily.
Rescue centres do an excellent job overall and without them we would be in an awful mess.
Many dogs are placed into care and the previous owner has been far from truthful as to the extent of the dog's behavioural problems. The dog can then be re-homed and after the honey moon period of 2-4 weeks is over, these problems can start to come to the surface. You may see issues that range from separation anxiety (common in re-homed dogs) to aggression to dogs or people, to non recalling!
When assessing a rescue dog you should consider a number of aspects that may affect you in various degrees. Key words are the way the dog is known or seen to react to...small animals, children, other dogs and members of the family both male and female.
It's a heart rending experience walking around your average re-home centre, so you will need to remain calm and collected as you pick a dog, as the surrounds will potentially want to make you rescue them all! Personally, I don't subscribe to the 'dog chose me' line of thinking. An owner should look to proactively choose a dog that suits the owner's experience, energy levels and other factors such as time and space available for the dog.
Rescue by all means, but stay calm and collected in the process. Do also bare in mind that there are many many rescue dogs looking for homes that are perfectly amenable and ready to share in your life.