can switching regulators cause interference on wifi based circuits?
Quick answer, as from Chris Stratton, is YES!

Longer answer: noise from electrical circuits is caused by currents, which turn any and every conductor into an antenna some some sort. The amount of noise they generate out of the unit will depend on the efficiency of the antenna (the length of the antenna vs the frequency of the signal) and the amount of current being switched. Buck and boost regulators work by switching currents through inductors, hence produce noise. A well designed switched converter will produce very little noise by the use of filters, shielding, well routed tracks etc). Meanwhile a linear regulator does not switch currents, it basically "burns off" the extra voltage, and so does not produce noise. However, if you want to boost the voltage, you pretty much have to switch currents in some form or another depending on the typology you are going for. 

You mentioned the efficiency of a linear regulator is worse than a switcher, which is very much the case. If you are using a linear to get 3.3V from a 5V rail at 100mA, 100mA goes into the LDO (plus a little bit of overhead to power the LDO's internal circuitry). The device then dumps the excess power as head, so you have 100mA, 3.3V out (0.33 Watts), and you loose 100mA, 1.7V (0.17 Watts) as heat. Power out over power in gives you about 50% efficiency. Meanwhile a good, synchronous, switcher will give you about 70-80% efficiency.

As for interference with WiFi, again, the answer is that it is possible, but only if very poorly designed. A switcher tends to work in the 100kHz range, a lot lower than WiFi's 2.4 or 5 GHz, however you also have to worry about the harmonics of the aggressor's signal. It can get very complicated, it is worth being aware of it, but there will be bigger issues than your WiFi dropping out if your DCDC converters are causing that much noise...