I've never used lantus, but here's what I understand. (Lantus users, correct me if I'm wrong)
Lantus is what's called a depot insulin. This means that a kind of reservoir of insulin builds up as you use it to keep glucose levels more stable - less highs and lows. If you miss a shot that reservoir gets depleted, making wilder glucose swings. Missing a shot on a regular basis negates the positive effect of the depot insulin.
How is your cat doing up to now on Lantus? Stable? I'm afraid regular missed shots will damage your current control. If life circumstances dictate that shots are going to missed on a regular basis, you might need to talk to your vet about using a different insulin. I think there are other, more forgiving insulins out there. Again, I'm not sure, but I think ProZinc might be one of them.