Is consistency in citation style important? I have a habit of using two styles of in-text citation: with and without author name(s). The two examples below describe what I mean: (# here represents a bibliography index)

"...which is in agreement with simulations presented in Lastname et al. [#]."

and just

"...the X method [#] was here used to model..."

(in the latter case, reference # might contain an in-depth description of the X method).

I mix these styles freely, depending on what I deem to be appropriate in each individual case. Am I correct to do this or would you consider it to be bad style?
To your title question

Is consistency in citation style important?

Yes!
On first reading this, it sounded as though you were planning to mix two citation styles such as APA and Chicago. Obviously, this would be unacceptable.
However, in your example, you are using two entirely compatible forms for your in-text citations, which is perfectly acceptable. I would strongly prefer this over the awkward and boring alternative of using exactly the same form for every citation. Continue to use the form that is most appropriate for the situation. Variety is acceptable, even commendable, as long as you are not violating the standards for your documentation style.
Edit: To clarify the point above; mixing the in-text citation style Lastname et al. [#] and [#] is OK! What is not OK is using two separate documentation styles, for example, also using the in-text citation (Last name, year, page), which is proper is APA documentation style, but not in Chicago documentation style!