Today I arrived in Seattle from Vancouver by bus with a fresh I-94 for 6 months. I'm leaving by train tomorrow and I know I'll be coming back to the States a lot of times before August. Do I still need to return my I-94 on every visit or does the US not care when exactly did I leave as long as my exit is recorded sometime before the I-94 expires?
Returning the paper I-94 isn't necessary for the US to record an exit at the Canada land border. Canada and the US exchange entry records for third country nationals crossing the land border, so your entry into Canada also provides an exit record for the US. The US will know you left whether you return the paper I-94 or not; you should be able verify this by looking for the records in your travel history at the I-94 web site.
Note, however, that even before they started exchanging those records (I think in 2014) a frequent traveller wouldn't return the I-94 at the end of every trip. You would instead hang on to it until it expired or (better) until it was close enough to expiry that you likely would need a new one for your next trip. I'm pretty sure they actually want you to do this since it minimizes your visits to secondary and saves them work. Since they started getting reliable exit records they now seem to be happy to let you enter even without the paper, so if you take a flight to the US that I-94 may get you entry on subsequent land border crossings without a secondary visit.
So if you expect to travel frequently don't return the I-94 unless it is close to expiry or you are planning to travel out of North America for your next journey. In fact, just keeping it and letting them take it when they send you to secondary to pay for a new one seems to work okay too. It used to be that if you didn't return the I-94 before it expired you would run a small risk of being questioned about when you last left on your next entry, but now that they get electronic exit records at the Canada border they don't seem to care much about the paper record keeping.
I live in Mexico and have I-94 stapled in my passport. It stays there for the 6 months.
If there isn’t one there (we lost them once) the border officer sends me to get a new one (including paying again).
If it’s there in my passport they check it’s current and let me in. Not an official answer, but certainly my experience of the past year crossing multiple times a month (entering the US with a B1/B2 visa). This is of course at a land border.
There has never been any comment made about having to return it. Your re-entry fixes your departure time (if a little inaccurate) so there shouldn’t be an issue.