You are right, there is unfortunately no commonly agreed upon or standardized way to model web sites and applications with UML. There is a plentitude of approaches with varying impact, from the early Jim Conallen with Building Web Applications with UML over WebML to UWE, as none of them can be considered authoritative, your best bet in deciding this question is to:
a) Look at as many of these approaches as possible and settle with what the mayority does *, and...
b) ...use some common sense.
One hint for each a) and b) from me:
a) There is a nice example of an UWE UML model here. This example does model the login process.
b) The login process is quite an important part of usage and activity on a site (say you have different content depending of if you are logged in or not, you might login with an 3rd site auth provider like OpenID, ...). if you don't model that - what will you model then? I think the people saying you should not model it do so mostly because they use notations that are not powerful enough, or they just don't know how to use them.
This, I guess, amounts to my advice: do model the login process.
* Provided you can still find them - some of them seem to be quite short lived