In some cases, yes, people just ask their doctor. And depending on the insurance the person has, this may work fine.
But it’s not necessary to ask one’s personal doctor. There are at least two businesses that provide prescriptions online that can be taken to a local clinics such as LabCorp. One convenient source is LEF.org, but shop around as there’s no reason to choose anything but the lowest price, as other labs that might be used won’t be worse than LabCorp.
While off-cycle, it’s good to have a baseline for free testosterone and estradiol. If free testosterone is poor, LH should be measured as well, to understand whether the problem is low LH or, if LH is good, poor testicular function.
During a cycle using aromatizing steroids, it’s good to measure estradiol so that dosage of anti-aromatase can be adjusted to reach a desired target of about 20-30 pg/mL. Some will have concerns for blood lipid profile and may wish to monitor that.
A CBC will allow seeing if the androgen usage is increasing hematocrit excessively. There probably really is not a sharp line, but 53% is widely considered a maximum acceptable limit. Blood donation can be a solution when close to the limit (to avoid going over) or sometimes when just barely beyond, but more usually reduction in dose is needed in those instances where hematocrit is excessive. AST (a liver enzyme) should also be monitored when using alkylated oral anabolic steroid.
I don’t think it’s necessary to do blood work every time, but it is good to have a baseline and to have known what’s happened in the past when doing cycles similarly to what’s currently being done. For example, if knowing before that a given amount of Femara Letrozole was what you needed for a given amount of testosterone, that same dose will be close enough for future use as well.