In the absence of an indication that premature delivery is imminent, I would not get the betamethasone shot.
My DD was born at 32w4d. I had received steroid shots at 29w, because of bleeding related to placenta previa and a concern that we might have to deliver DD emergently then, however, DD and I hung in for another 3 weeks, and DD had major breathing problems at birth. At one point, her respiratory distress and the issue of the steroid shot came up and the neonatalogist said that, in his experience, 3 weeks before delivery was too long for the steroids to be helpful. The info I can find on WebMd is that "Betamethasone or dexamethasone is most effective if delivery occurs at least 24 hours after the first dose of the medicine has been given and less than 7 days after the last dose of the medicine." From that same article - "If delivery does not occur within 7 days of treatment, the injections should not be repeated."
(Article here: http://www.webmd.com/baby/antenatal-corticosteroids-for-fetal-lung-development)
And then there's this, a summation of three Cochrane reviews on corticosteroid use in pregnancy, which includes "No benefits were demonstrated for treatment commenced, or infants born, before 26 weeks of gestation, nor for those born more than seven days after treatment. " (my emphasis) (http://apps.who.int/rhl/pregnancy_childbirth/complications/preterm_birth/cd004454_hofmeyrgj_com/en/index.html)
So yeah - my experience, and these pieces of research suggest that corticosteroids really aren't great for prophylactic use. I wouldn't get the shots unless there's a real indication for them.
Although, hang on a second, you say "babies", and this is the Parenting Multiples forum - how many babies are you carrying? Multiples carry a much higher risk of pre-term delivery, which may qualify as an indication. However, given that the benefit from the shot isn't, like, permanent, I would want to delay shots as long as possible, to match up delivery and the benefit period as closely as possible.