Even if the '157 is a make-before-break switch it happens so fast there's zero chance of damage, even with long-term use.
Chips aren't real switches, at no point do they actually switch a physical connection like a normal, hand-operated switch does. Chips are more like a virtual switch, where they produce an output identical to the input, but from a different source entirely. Put another way, when you adjust the select line of the '157 it adjusts its output (1Y) to match the input of either 1A or 1B, it does not physically connect these inputs to the output.
Conclusion: Stop using your antiquated analogue concepts when dealing with the exciting new world of Digital Technology <insert booming sound and angel choir here>. Also, don't worry. =)