I think the right time to get something checked is when you are worried about it - I'm a firm believer in Mothers Intuition

At 5 months I took DS to the Paed as I was worried that he would not lift his head when on his belly, something just seemed wrong to me.

He was diagnosed with low tone and hip dysplasia after ultrasounds and xrays which thankfully is all corrected now (he'll always be a bit on the floppy side with tone, but perhaps he might be a trapeze artist or a flexible dancer hehehehe)

Most likely there is nothing wrong, but wont you just kick yourself if you find out months later you were right? A quick and painless Xray/Ultrasound would be all that's needed to check if there are any concerns with the legs etc

Good Luck either way