There are so many variables for a standard answer as it really depends on the mic, the room, the singer.

If what you have been doing works, then it works. You have to know what it is you like and what it is you dislike to have a reference before you can make any changes that are meaningfull to you.

I approach it from the aspect of knowing very well the sound and character of all my mics, mic pre's and what the recording space can do then match that to the singers style, song arrangement and musical statement your trying to achieve. Once you have all that worked out, then it is a matter of using and controlling the variables to achieve those goals.

By variables, things like mic sensitivity, which polar pattern I am using, how many mics I'm using, is there any rumble or do they move their head around a lot or tap thier feet, how sibilant the singer is, how well they can control their vocal and it's dynamics, or how much mouth noise they make, how hard they have to strain to hit the high notes, where the lyrics are placed for them to read, how much room sound do I allow into the mic, how much leakage am I picking up from headphones or other instruments, are they playing an instrument and singing at the same time. These are just some of things that dicatate mic placement.