[OS X TeX] Aligned equations (OT)

[Themis Matsoukas]

Following advice from this list I started using the align environment  
instead of eqnarray. Even so, a problem I have encountered is that  
pages with many aligned equations are often typeset with enormous  
blank spaces, since latex does not break aligned equations (this  
happens with eqnarray, too). By replacing a block of aligned equations  
with individual equations this ugliness goes away. Hence a question of  
style:

When is it stylistically (im)proper to align equations? I have always  
aligned whenever I had a bunch of equations with no text between them,  
for example:

... we define the following variables:
%
\begin{align}
  M_* &= \mbox{characteristic mass}
  R_* &= \left(\frac{3 M_*}{4\pi \rho}\right)^{1/3}
  V_* &= \left(\frac{8 k_B T}{ \pi M_*}\right)^{1/2}
  D_* &= \frac{k_B T}{6\pi\mu R_*}
\end{align}
%
...etc etc

but is this a valid rule?

Thanks

Themis Matsoukas
matsoukas at engr.psu.edu