Today I read that flash write performance diminishes with use. I'm a heavy user. Should I change my behavior?
As a user, and not a developer, this is not an effect you should concern yourself with. For operations like disk defragmentation or use of flash memory as virtual memory, the sustained high disk write rates can wear out flash memory quickly. But "normal" usage by a human will not produce enough writes to create a wear issue. 

Let's take the sleep app as an example and estimate how many writes this will cause on a wear-leveled disk. Suppose the app collects 128 kilobit / second audio to store uncompressed and runs and collects audio at all times for 5 years. (128 kilobits / second) * (128 bytes / kilobit) * (5 years) * (365 days / year) * (24 hours / day) * (60 minutes / hour) * (60 seconds / minute) ~= 5.17 * 10^11 bytes. On a wear leveled disk of 1 GB ~= 10^9 bytes, this will be (5.17 * 10^11) / (10^9) ~= 517 cycles over the 5 years, which is well below the expected 100,000 cycles for current flash memory. 

Also, note that almost all SSDs are currently built from flash memory, so there is no significant difference between the storage for your Android phone and that in a consumer SSD for a laptop/desktop in terms of expected aging issues. 



EDIT

If you are really worried about this, I would benchmark your storage performance. I don't think it is a good assumption that erase/write operations take 0.6 seconds; your flash memory is not the same as that used in the example figures. 

This empirical evaluation of flash memory gives actual figures for degradation. The magnitude of the effect is not that large for cycles in the low thousands.