Choose proper grounding when using multiple LDO, buck, buck-boost and invering regulators
Ground loops are a problem for a few reasons. Loops, like you mentioned are one reason, its a great way to turn the design into an antenna. This is bad if you are trying to pass an FCC regulation or if your design is susceptible to EMI. Minimize the loop area as much as you can. If EMI still becomes a problem, try increasing the increasing the inductance of the loop on the non ground side with ferrites to block high frequency noise. 

A bigger problem can be common mode noise.

If you look at this figure from Electromagnetic Compatibility Engineering by Henry W. Ott, you can see why this would be a problem. The current from the 'upstream' regulators is going to bump up the voltage on the downstream regulators.

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A bigger question is, will this effect the design? It depends on the currents and the impedance. It also depends on what other circuits you have, and if they are sensitive to grounding changes. If you are trying to measure millivolts to microvolts then you might have trouble. If your circuit is purely digital, then some common mode noise might not be a problem. Use equation 3-4 (come up with a similar equation and diagram) either measure the ground noise or it can be found in some datasheets.

If the ground currents are not varying, then the voltage common to both of the regulators will be constant. If it is constant, it may not be a problem. (this would be unlikely, I know of few circuits that draw constant loads these days)

Minimizing the impedance of the wire between the regulators will help by reducing the voltage from the resistance of the wire. Changing the wire size to a larger size would help, reducing the wire length would help. 

Swapping the order of the devices in series can also change the current flowing through each section of wire, which might also help. 

If these options fail, then you may have to switch to a muti-point ground or a combination of the two.