I've seen people enter for a $2 DLC on an $80 bundle, and that's only the highlight of a long series of self-centered entries. It's less common at levels above 6, as you have this giveaway set to, but I've certainly seen it at every level range. "Trust but verify" seems an apt idiom to go with for this site.

That said, packages [ever since Steam distinguished between bundles and packages, the distinction in naming there has become notable] aren't all that much a hassle to deal with. In fact, they can be really comfortable:

1. SG requires GA creators to give what is promised. The major exception to this is packages, where the GA creator can set any related rules for entry they wish [ie, allowing the setting of rules which are based specifically and only on the consideration of ownership of the elements contained within the package, for any entrants with at least one element already owned]. This giveas you a lot of freedom as far as how you want to present your giveaway- a freedom usually restricted to private or group giveaways.
   (Note: If an element contained within the package is exclusive to the package, then this exception may not actually apply, as the package will then usually be considered by staff to be its own separate listing, rather than a packaged listing).

2. You can easily check ownership of base games by checking a user's library and, while a bit more of a hassle, you can add the user as a friend and go to the store page for any DLC to see if they're in your "Friends who own this game" section for that DLC. So, verification isn't that much of a hassle.
   In the case of private profiles, you don't even need to ask them to go public- SG staff has access to sync records [ie, this page for any given user] and can check the info for you. They can't, however, check DLCs for you, and any content removed or otherwise unlisted on Steam can't be checked at all. So that's the point where packages do become a bit of a hassle.

Well.. Hopefully those lumpy paragraphs didn't increase how daunting the matter comes across to you. :S

Also, to clarify (and partially correct) the "the site can't detect correctly if you own everything or not from it" that LevTaylor noted above:

Steamworks uses two login systems. One is the developer system, which allows developers and website owners to pull public information from a user's account. DLCs information isn't made public to that system, and therefore a site using that system (like SG) can't dtermine ownership of any content which contains at least one DLC. However, in the case of a package containing only base games, ownership can be detected- similarly, SG is configured to note which games have been previously won on SG (as that information is already stored locally on the site).

Meanwhile, the user system is used by gamers to log into games, or to log into Steam Community, or into Steam Community enabled websites, and grants full access to the user's data. This is why scripts and sites iike IsThereAnyDeal can pull up information that SG can't. However, that system requires regular logins, and that fact is likely the reason SG doesn't have any integration for it.

In short, unless a package contains DLCs or is misconfigured, you don't have to worry about anyone who owns 100% of the content entering the giveaway. There are other circumstances to note, however- Steam doesn't send ownership information out properly on removed games, Profile Features Limited games, or games that've been released within 24 hours- though that last one may be wholly superceded by the PFL aspect, now.

Moreover, you'd still have to decide how to approach those containing more than 0% and less than 100%, however, as per my above comment. So, uh.. yeah, giveaways can sometimes be a pain. Definitely makes you appreciate more those users who put up giveaways for DLCs or the like, however.)