Assuming my memory isn't failing me, I believe the claim may come from Fernando de Alva Ixtlilxochitl. This could be a problem. Ixtlilxochitl was essentially a propagandist for the Texcocan nobility, and he made some dubious claims on their behalf, including: that Nezahualcoyotl had been a proto-Christian who opposed human sacrifice, that he created all the best laws of the land, that the Mexica sacrificed tens of thousands of children (and so Texcoco wasn't responsible for it), that originally Texcoco had been head of the Triple Alliance only to be usurped by Tenochtitlan later, and so on. However, some of his claims do appear to be based on some accurate data. For example, he maintains that Nezahualcoyotl was an exceptional engineer. Lo and behold, the Imperial Tecpan in Texcoco has some of the most sophisticated hydrological engineering in Central Mexico.

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So, is this claim true? It is possible. Most late Aztec period states attached a high value to market exchanges and merchants. Placing an easy access food reserve on the road for travellers, most of whom would be heading to market, would be a logical way to facilitate trade. Merchants could travel to markets without having to worry so much about the journey. Thus, they could come from farther away, and bring more goods. Furthermore, it would be easy to carry out. Most Central Mexicans paid taxes through labour. Thus, it would be easy for a lord to instruct his workers to plant a few crops by the local road, and to look after them. Another, easy to overlook factor, is yield to seed ratio. Each seed of maize yields a vast number in return (compared to say wheat or barely), and so diverting a small portion of seeds to line the roads came at very little cost.

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Where I disagree with Ixtlilxochitl is that I believe this practise was probably common throughout Central Mexico, and possibly predates the Mexica Empire. Assuming of course, that I've remembered the source correctly.