I prefer them specially with infants who've never been in group care before ... as mentioned both for the PARENT piece of mind and than so that the first day is not a 9 hour day of 'screaming' if they are a screamer .... however I generally prefer them to all be consecutive days typically the week or two depending on how much time we have before they have to 'start full time' increasing the time left here alone longer as they are doing 'better' ... so first day plan a drop off from like 8am stay to 10am type thing and next day do a little longer and so forth.

Older kids I typically do 1-2 play dates with the parent present and than start cold turkey cause they are easier to 'reason' with that a infant is!