
I've never chanced having boosters on lactating bitches myself.
I had a bitch done a little early when I knew I was going to have her mated. Booster was done on day 2 of season, and she was mated on days 8 & 10. She always stands early, and has very short seasons. Flags from day 3! She had a perfectly normal pregnancy, and her litter was fine.
I also let a girl go overdue slightly, and had her booster done when pups were 10 weeks.