How close can I place 0603 resistors?
I have a silk layer bounding box of 2.6mm x 1.4mm around the pads of 0603s, and I often place resistors with touching bounding boxes. Lengthwise placed next to each other this leaves 0.2mm between pads. For reflow soldering this has never caused problems. For wave soldering you will need more space, esp. in the wave direction. For hand soldering it depends on the soldering skills of the person who assembles the board. 0.2mm may be possible if you don't use too much solder.  

While 0.2mm may look like very tight, remember that this is reflow soldering. When the solder paste melts it's capillarily drawn to the contact surfaces of the resistor, so it won't flow to adjacent pads.