I’ve been asked this a bunch of times by prospective Nepal travellers over the last couple of years. The answer is an overwhelming NO! There is almost no way to get lost on hikes like the Annapurna Circuit or Everest Base Camp. The trails are large and well marked. If somehow you’ve managed to lose your way there will be people to ask for help. You’re not hiking alone on some vague and forking foot trails, it’s more like a highway; thousands and thousands of trekkers do these hikes every year, as well as sherpas and yak teams.
Also when you have a guide, he likes to be in charge. He will usually have a well laid out itinerary for you that includes all the guesthouses, whos owners he’s friends with. Say you are feeling good and want to keep going to the next village a few hours away. Well he might not exactly share your enthusiasm. Also English is another consideration. If you are going to get a guide, definitely wait until you get to Nepal. I’d want to meet him in person and feel him out a little bit over a coffee before agreeing to his services. Remember you are going to be spending two weeks in the mountains with this guy, so his making sure you guys can communicate well is pretty important. If you feel good about the experience and added level of safety he will bring to your trip, then certainly go ahead. But for someone like me who is happy to do some research on the internet beforehand (and has some hiking experience) I find a guide largely unnecessary.
Concerning porters, if you’re in your twenties or thirties and in reasonable shape (not a fatass), then I really don’t think you need a porter either. You can pack fairly light for the trip, as you’re sleeping in guesthouses you don’t have to worry about packing stuff like a tent, stove, food, excess water, etc. Enough warm clothes, medicine, water purification tablets, headlamp, and some sort of entertainment are the important things on this type of trek. Then after you’re satisfied with your bag’s weight you can throw in a big ass camera, 2 lenses, some filters, extra batteries, and tripod to really fill it out. Or that’s just me...
Now if you think your personal enjoyment will be that much greater carrying around just a small daypack, than by all means go for it. But if you’re male especially, you might really feel like a wuss when you won’t carry a 20lb bag while these little Nepalese guys are hauling around a hundred and something pounds. Ok, you shouldn’t compare yourselves to these guys, cause they’re freaks, but you probably will. But you can compare yourself to me. And if you think you're wussier than me, than go ahead and take that porter.
At the end of the day for me, choosing not to take a guide or porter isn’t about saving money, or some self-sense of machismo, it’s really about independence. I like to feel like I’m free high up in the mountains. Just me, the white jagged peaks, the bite of the wind… and that slow ass Danish guy a couple hundred feet behind.
Quick Note: As for crossing the high passes in connection with Everest Base Camp route, the trails are not so well defined. Know how to use a map and compass at the very least. For us the trail was easy enough to follow and the weather was good, but if you get some snowstorms and poor visibility, you could definitely get into some problems.