Recently I read this short piece about whether it's okay to mix past tense and present tense in your writing, and my inner linguistics geek stood up and started stomping her feet, so here I am.
Let me remark something about grammar:
The effective use of grammar is not about what features of it appear on any particular page. It is about what the choice of a particular form allows you to do.
I hear the phrase "mix past and present tense" and I blink. What does that mean, "mix"? Does it mean, just write along and don't pay attention and whichever one comes out is okay? Well, then I'm entirely against it. On the other hand, I have written an entire novel which uses a diarist's point of view, and in her diary she discusses things that have happened to her - in past tense - and things that are going on at the time when she's writing, including things happening around her and her assessment of people's current qualities - present tense. In early drafts I had a couple of readers, confused by the unpolished prose, call me on "tense-mixing" - but it wasn't tense-mixing, it was just that I hadn't shown enough of the setting for the current ongoing events part, and so the proper context for the use of present tense wasn't clear. Once I properly established that, the problem went away. My use of verb tenses didn't change at all.
Example from Through This Gate (Dana writing in her diary about trying to figure out her new roommate Shannon):
Maybe mom was hinting that Shannon has some kind of granola-head thing going and I shouldn't let myself be influenced, but I'm not sure that fits with the makeup, or the computer either. Anyway, when the last box was in, Mom looked around my empty half of the room as if she didn't notice the bare blue mattress or the battered furniture. "This is great," she said, gesturing - I swear, the woman could conduct orchestras.
There are a lot of "traditional" past tense narratives out there in the fiction world. We grow up with them, and because they are the environment we're steeped in, we've long since stopped finding the use of past tense remarkable. On the other hand, if you're really paying attention, I think you'll find that all these past tense narratives also contain uses of the present tense - you'll certainly find them in dialogue and direct expressions of a character's thought. I hope you haven't been thinking that those examples of present tense in a past tense narrative "don't count." Sure, they count - they are in present tense precisely because they are doing something different from what the rest of the narrative is doing. If we were listening to a narrative read aloud, the tense (along with prosody and dialogue tags) would be a major indicator of when we were listening to dialogue.
Example from The Once and Future King by T.H. White:
Kay looked at his father. He also looked at the Wart and at the sword.
Then he handed the sword to the Wart quite quietly.
He said, "I am a liar. Wart pulled it out."
We also shouldn't forget that we change our verb tenses all the time when we narrate stories verbally. We'll be in the midst of recounting something that happened and when we get to the crux of it, we'll switch into present tense to place the listener more inside the moment when that exciting thing happened.
Example: "I went to talk to my boss about it yesterday, right? So I'm walking in there and I say..."
Honestly, I'm not sure this one works effectively in written narrative - but I do think that it is realistic to have such use of verb tenses in dialogue when one of the characters is engaged in that kind of storytelling.
I've also seen tense used what I might call "aggressively." The term is an exaggeration, but what I mean is, the tense gets deliberately changed for a particular effect. In her book The Handmaid's Tale, Margaret Atwood begins in present tense, creating a dreamy effect where there's no sense of the passage of time; then, as the main character's viewpoint changes, she switches to past tense and suddenly the story begins to achieve a sense of momentum. It's unusual, but it's deliberate, and really cool. As for me, when I'm working in alien point of view, I deliberately choose present tense, and I do it so as to force the reader to align more thoroughly with my alien's impressions, emotions, and judgments. I've been told my alien point of view stories are "challenging, but worth it." The fact is, present tense gives me a kind of intensity that I can't achieve with past tense.
Example from "Cold Words" (Analog Oct 2009):
I scent human outside the door: our linguist, Parker. He never comes to the Ice Home while I attend Cold Council - he must bring important news! I bow to haunches, then excuse myself from Majesty's presence, quickly as I can without inviting snarls from the others.
So I guess I'd conclude by saying I don't think it's okay to "mix" present and past tense - because that implies a lack of care and precision. It's perfectly all right, however, to challenge yourself and your narrative, and your reader, and use whatever verb tense you need in order to serve your own purposes.