Been having a recurring problem lately. The bolts on the left crank arm keep coming lose. Should I use locktight? Suggestions?

Have you tightened it with a torque wrench? The non-drive side loosening up on SRAM cranksets seems to be pretty common if it's not tightened to the maximum of the range specified by SRAM. I had it happen on one of my Rival cranksets.

Same here, last week I had to ride last 20 km with a wobbling Rival crank.

Likely undertorqued as mentioned. If you don't have a lot of experience wrenching...even if you do...I generally use a torque wrench on cranks...invest in a good 3/8" drive torque wrench. Doesn't have to be high end wrench like Snapon...Craftsman will suffice. Also good to own a set of socket allens....hex allen bits attached to socket which plug into a torque wrench. Sometimes torque target isn't achievable with L shaped allen due to lack of leverage...or owner doesn't know the torque target. Your post is unclear...you may mean the single center crank bolt...depends on the crank...but if you are refering to left crank arm dual pinch bolts aka Shimano...torque in thirds...set torque wrench 1/3, 2/3 and full torque spec and alternate bolts for uniform clamping. Torque specs are generally found on the web...most crank makers have service manual PDF's on line. I always use a dap of grease on the spline to prevent bonding. Inspite of lubrication, this actually helps clamping strength...grease is captured in micro-voids between male and female splines improving clamp strength.

Look closely for cracks or other problems. I had an old square taper that came loose. Realized the left arm had a crack that spread open as it was tightened.

Also, lubrication of bolt threads can decrease the torque requirement from spec. Torque specs are usually for dry threads.

Actually, I didn't say anything about lubrication of bolt threads...just light coating of grease on the spline. I however always add a small degree of grease on bike threads including crank fasteners. And quite right, this does bias the torque spec..this debate in fact rages on many forums about this very dynamic...in particular, maginitude of influence.
I will tell you what I believe to be most problematic which it comes to adding grease or anti seize to cranks. Older square taper Campy cranks were 'extremely' sensitive to adding grease to the square taper. This would not only affect the center bolt torque spec, but how far the spider or non drive side arm would draw onto the spindle. To me, this was a fundamental weakness of square taper. Another volatile debate pertained to whether to add grease or not to the square taper...a matter of philosophy. Many who didn't, ended up with creaky cranks.

I use a dab of locktite thread locker in blue on stuff that could back off and make me fall typically.

Use loctite blue. I recently had an FSA crank come apart on me during a ride. One of the crank arms just dropped...