Here is my answer: Yes, it is ok to warm-up daily. As long as your vocal warm-ups are not too strenuous and don't last too long (15-30 minutes is ideal). In the same way that it's ok to do some type of physical exercise each day, it's ok to warm the voice up. However, you might benefit from a day or two off as well.
As for my personal schedule, since I am on summer break from teaching, I currently warm-up on the days that I teach lessons, lead choir rehearsal, and on Sundays before singing in church. That adds up to 4-5 days per week. The other days I'm usually not singing, so I choose to rest my voice on those days. When school starts up again in a few weeks, I will warm-up Monday-Friday along with my students (efficiently killing two birds with one stone!) and perform a more strenuous warm-up on Sundays before church.
The purpose of a warm-up is two fold: it should ease and prepare your voice for heavier singing and it should help you practice good vocal technique. Use these guidelines when deciding which warm-ups to practice. You don't want something that is so difficult to perform that it causes any strain whatsoever to your voice. That, in my opinion is not a true warm-up. My students always comment that their voices feel and sound freer after performing warm-ups. That is what your voice should feel like after a gentle warm-up session.
Three to five warm-ups per day is usually a good number to perform before diving into heavy singing or practicing your repertoire.
Proper warm-ups will keep you from sounding like a frog!