Saguaros bloom from early May into early June, but this varies a bit with the spring rains and temperatures. Early heat (approaching 100's or into low 100's) means an early bloom. Keep track of daytime temps. This having been said, the earliest blooms appear at the lower elevations. Around Tucson there are good concentrations of Saguaros at Saguaro Monument West (in the Avra Valley past the Desert Museum), Saguaro Monument East (east side of town toward the Rincon Mountains, south end), The Reddington Pass Road into the Rincon Mountains, Sabino Canyon and Mt. Lemon Highway. Watch out for snakes.
One problem is ground contour. For really good close shots, you need to be 10' tall, have a ladder, have a long lens or find a hillside where slants take you close to the tops. If using E6, put highlights at 1.5 stops above zone V. Use an 81b filter for cool sky tones. For B&W place specular highlights at zone VIII or IX. tim