Not sure that I agree with Jane and Stewey on the necessity of regimentation in caring for our extra sweet kitties. Some people have different work or social life schedules for different days of the week. You can certainly work around these variations and still give your diabetic cat good care. There are members here on the board that do what works for them. ECID doesn't only mean Every Cat is Different, but also means Every Caregiver is Different.
Some of the insulins are more flexible in the shot time, not exclusively restricted to 12 hour cycles. Prozinc has more flexibility in the shot times. Even some Lantus users have employed non-traditional cycles such as 13/11 or 14/10. Just ask Larry & Kitties.
Testing at slightly different times in the cycle can actually be very helpful in seeing how your cat is responding to the insulin. Yes, we always encourage those pre-shot tests to make sure your kitty isn't too low to give insulin in the first place. But after that, one day getting a +4 and a +5 and another day get a +6 and a +7 can help to fill in the spreadsheet and build up a better view of what the pattern is for your kitty.
There are some factors that are within our control and others that are not. For example we can't always get our cats to sleep the same amount each day or exercise the same amount each day. There will be variations in the amount of food your kitty eats at one sitting.
So while consistency is helpful, there will be times that other commitments in your life necessitate flexibility in the care of your diabetic cat.