Just as article 551.71 makes no mention of needing GFI protection for 50 amp RV receptacles, article 630 makes no mention of 50a welder receptacles needing GFCI protection, at least in the 2008 NEC. Chapter six installations modify or supplement previous chapters per 90.3. You may not need GFCI protection on the 50 amp circuits, check article 6:30 in the 2017 NEC to see what it says. As far as nuisance tripping, in the 08 NEC under 630.15, the fine point note, if your welder secondary is connected to a grounded object, you will create parallel paths and objectionable current, you will also trip any GFCI device protecting the primary.
I'm sure most welders know this, however with arc welders you put the clamp on the work, not on a grounded object. Point is to allow the electricity to return to the source, the secondary side of the welder transformer, and not to the primary side, which will more than likely cause nuisance trips.
Are all 5 welder receptacles on their own circuit breakers? If not, and they are trying to run two welders at once, it will not take much leakage at all to hit 6 milliamps and trip the breaker on ground fault.
So I guess the choices are one, you do not need GFCI protection, two, you do need GFCI protection and have to figure out if all welders are tripping, if one or more of them has defects causing ground fault trips. You may also want to check the owner's manual or installation manual, they may not be compatible with GFCI protection / manufacturer instructions. In that case, 110.3 b maybe your friend.
Edited to add... I did a precursory search on the internet for running welders on GFCI circuits. There are some conflicting reports... It seems you can run Mig/tig welders on GFCI just fine, arc welders perhaps not.
Member augie47 ask a very similar question on the same topic perhaps a week ago... Not sure if he had any luck in resolving his questions / issue... May want to send him a p.m. or maybe he will chime in on this thread