Some very experienced users will still give a Lantus shot when their kitties are above 50. However, shooting that low means that you'll have to get some mid-cycle ("in-between") tests in. If you can't get mid-cycle tests, aren't going to be home to monitor, etc., then I personally would not recommend that you give a shot.
Lantus dosing is based on the nadir (the low point in the cycle). If you don't get some mid-cycle tests, it's hard to figure out if the dose needs increased, decreased, etc.
That's just my 2 cents' worth! ;-)
Shelly