It is rare to get a good ultrasound at 5 to 6 weeks, usually you will only see a sac and maybe a fetal pole. I would wait until week 8 or later when you will be able to see a heartbeat.
Another reason for an early ultrasound would be a history of ectopic pregnancy - in this case the US could confirm that the embryo is located where it should be.
If you should go in early depends on the circumstances of your previous miscarriages. Usually doctors will only consider additional testing/prenatal visits if you had at least 3 aborts at similar stages in the pregnancy or for similar reasons, if the reason could be determined. At this early stage of pregnancy a series of blood HCG levels are more indicative of a viable pregnancy than an ultrasound, so you could consider that.
Also if you have previously lost pregnancies due to low progesterone (a blood test can determine this) you doctor may consider putting you on a progesterone supplementing therapy. This is not terribly common though.
Other than that there is really not very much that can be done to avoid early miscarriages.
I know that people do not like to hear this, but the majority of early miscarriages occur because the embryo did not properly develop. Usually these embryos do not have the capacity to develop into a viable fetus even if we had a way to stop a miscarriage.
I would suggest that you call your doctor and simply ask when they would like you to come in. Good luck!