I typically turn on CSS when teh tool is at the start point of the path. Just a habit.
G99 or IPR is a default mode, and is usually reset by an M30, so likely that it isn't needed at all. Good safety measure though at the beginning of the program.
So, here is how I use the toolchanges and the CSS mode.
Beginning:
G54
/G28
G50 S1600 <----- Spindle clamp, VERY IMPORTANT!!!
G00 G97 T101 S600 M03 <---- S is set to an approximate value
G00 X1.05 Z.05
G96 S360 M08 <---- CSS and coolant on here
blah
blah
blah
G97 S600 M03
M09
G00 G53 X-nn Z-nn <----Move to toolchange position with spindle fixed at 600 RPM.
Next tool codes....
....
....
Note: The G50 at the beginning is modal, and will apply to not only the rest of the program, but also to MDI as well. Make sure you always set that at least once in the beginning!!!
I like to have a fixed spindle speed during toolchanges, hence the G97 S...