i run an agency, so i'm talking about aggregate numbers here...
my per account spending has grown a lot, but my targeting has narrowed considerably - i'm on a never ending quest to improve my clients' ROAS and reach.
my accounts aren't controlled by budget limits, but by return. so as long as ROAS makes sense, the calendar days of the month don't mean much to me.
like many advertisers, i often use G's tools to find more places to advertise, finding that contextually relevant sites / pages work best.
when i search those tools looking for sites, i often have issues with targeting sections of sites, because the site owner on the other end has a hierarchy, but often doesn't comprehend that obvious segmentation on their end, is the key to getting people to bid the highest for certain pages. if i sell football helmets, i bid higher for your pages and sections about helmets. but if you don't segment your content well, then i may not be able to target it well, or see that it's available to target by itself. contextual targeting often solves this issue for me, but not always. further, contextual limits my reach to just those pages that are exactly relevant.
here's an example... say i sell KISS figurines... and your have a rock and roll music forum... i'd like to target the /kiss/ section, and i hope you list it on your end, making it show in my searches. if i contextually target your site's pages, two bad things happen...
1) my ad shows on your Bay City Rollers page when someone there posts about how much better BCR is than Kiss - and i don't sell on that page...
2) in your /kiss/ section, there are many pages there, where my target buyers are hanging out, and many of those pages are peripherally related to KISS, like maybe a bunch of pages of the cars that the band drives... and contextually, i'm a mismatch, and i lose exposure.
if you can add a new layer of attention to your adsense thinking, and if you add lots of sections that aren't identical to your contextual structure, you will allow more bidders to chase you, from more angles. and in an auction, more bidders means higher prices, for you.
so don't worry about me running out of budget at the end of the month, concern yourself with being easier to find, not as a site, but as many site sections (aka channels). not only is this one key to me making money, but the same applies to you. and if i were ever budget limited, the most relevant (and converting) traffic would be the last one i paused as i neared my limits.