The menstrual cycle is divided into the follicular phase followed by ovulation and the luteal phase. The stimulation of ovaries in IVF is carried out usually in the follicular phase. The old concept is that follicles are recruited in the beginning of the cycle and ovulation occurs in the middle of the cycle. Though a large number of follicles start being recruited in the early part of the cycle and hence stimulation in the beginning of the period is considered to be normal.
There is evidence from cancer patients where stimulation is done randomly and also from research where follicles seem to be recruited at different times. There is some evidence that some women recruit follicles in the luteal phase better than in the follicular phase. Please also note that the recruitment window (that is where follicles can be recruited) is longer and wider in the luteal phase is increasing the possibility of recruiting more follicles.
It has been proved reasonably well that egg quality does not worsen whether you stimulate in the follicle or phase and luteal phase. This gives an option of better stimulating protocols which will allow for a better response and possibly more eggs and embryos. Any luteal phase stimulation or random stimulation necessitates freezing of embryos.