No, all of Arizona isn't warm all the time. There are a lot of mountains and high country that gets very cold. there can also be quite a bit of snow at times. In fact, the north rim of the Grand canyon is closed from late October to mid-May because of the likelihood of road closures due to snow (and the cold).
check out this web site for the grand canyon.
Click on "more" at the bottom of the narrative and you will be able to get a weather predicter for the days you plan to visit. You will need to click on "weather averages", then a new window will open and you enter where you will be going, when and for how many days. I used March 15-17, and it said the "normals are lows of around 24 and highs around 51. Here is the link.
I agree it depends on how much time you'll be spending in each place. If you are going to spend a big chunk of your trip in Phoenix, definintely don't go in June. If only a bit of it is in Phoenix and most of your trip is up north, June would be ok. You'll just have to spend a lot of time in the pool or indoors.
thanks everyone! Decisions, decisions!!
Many NP's are even more beautiful with snow than without. GC is one of them. Those clear, cold, blue skies (I hope) and the overlooks are magnificent. And Phx is also good to visit that time of year so I would vote for (end of) march too.
One consideration for you: because March in Phoenix is generally pretty much perfect weather, you will find high season prices for most (particularly nicer) hotels. The hot weather in June brings summer specials at a number of area resorts (sub $200 instead of $400+).
You are all forgetting a very important factor when trip planning. Precipitation. I know that is the desert (High and Low) but you should consider that according to the Monthly tables on Weather.com, March is the wettest month for both Phoenix and Sedona and June is the driest.
That said, I made the trip once in March and never saw any rain. Grand Canyon was windy and in the 40's.
I say March because the mean temp and percipitation are fine. Actually, in Sedona you may run into snow which is magnificent!!!!
Phoenix area is wonderful at this time and Sedona can be because of the snow. (We get a lot of snow in March and April.)
As a mountain bike tour operator who is on the trails nearly every day I can tell you June tests my limits of dry heat. We close up by the end of June as it can be very dangerous to be outside.
My two cents!