Hi there,
Personally, I only use calcium with D3, always have and most likely always will. Some disagree with that based off of the thought of over supplementing D3, but that is extremely unlikely. And to overdose on D3 using a calcium supplement, you'd have OD'd on calcium multiple times over before you started seeing issues from Vit D toxicity.
IMO, it's a nice safety net to have to just always use the D3 version.
The theory against that is the new, stronger bulbs produce strong enough UVB that the dragons body will be able to synthesize enough d3 on it's own, and that supplementing will introduce too much into the system. I have never seen this to be the case in my own dragons, and I've never seen a confirmed case of Vit D toxicity here either.
FWIW, if we assume (and it is an assumption) that artificial UVB works at least semi-similarly to natural sunlight, and that a dragons process for synthesizing vit D from UVB is similar to humans (the process is more likely to be similar than not) then we can rest assured knowing that the body will only synthesize Vit D from UVB when it actually needs it. And once the body has enough D3, it actually destroys it in the skin, instead of allowing more and more to be absorbed and created. So with that line of thought, if you are using D3 supplements and your dragon is getting enough, it simply will just not produce anymore itself from it's UVB exposure.
However, if your UVB isn't actually strong enough to allow for enough D3 to be synthesized in the skin, and you're also not giving D3, then there is a much more real possibility of your dragon being low on vitamin D.
TLDR low vit D is way more likely than ill effects from too high of a vit D level in the body.
-Brandon