Does it make sense to assign functions like is_paged() to a variable rather than using it multiple times?

So, all query conditionals (is_paged, is_singular, etc) look something like this:

function is_paged() {
    global $wp_query;

    if ( ! isset( $wp_query ) ) {
        _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1' );
        return false;
    }

    return $wp_query->is_paged();
}

Globalize $wp_query and call the corresponding method of the WP_Query object. In this case:

<?php
function is_paged() {
    return (bool) $this->is_paged;
}

The only database hit you incur is from when the WP_Query object is created, which happens anyway.

Calling a function incurs a cost, so does using an if statement. Those costs on today’s hardware are nothing to really worry about.

That said, if you find yourself doing a lot of conditional checking using the same function, you’re probably better off to evaluate the control flow of your program.

Something like this:

<?php
if(is_paged() && 1 == get_query_var('some_query_var'))
{
   // do stuff
}
else if(is_paged() && 2 == get_query_var('some_query_var'))
{
   // do other stuff
}

Could become:

<?php
if(is_paged())
{
   switch(get_query_var('some_query_var'))
   {
      case 1:
          // do stuff
          break;
      case 2:
          // do stuff
          break;
      default:
          // do stuff
          break;
    }
}

As a final word of advice, I would “cache” results of conditionals (or any other function call, within reason) within the same function if you need them more than once, but I wouldn’t cache something like a object property unless you really need that persist from method to method.

Here’s a terrible example that (probably) doesn’t follow my advice above.

<?php
function wpse65916_stuff()
{
    $paged = is_paged();

    if($paged)
    {
        // do stuff because paged
    }

    if($paged && /* some other condition */)
    {
        // do stuff
    }
}