How deleterious is topping-off my Li-ion batteries (phone/laptop) every time they drop to 90%?
Yes, fully charging and discharging the battery will result in lower lifespan. The difference is not 10 times, but it is significant. 

Here are some example plots from Kokam. For the 80% and 20% tests every 50th cycle was done at 100% (the vertical red lines) to check the full capacity.   

At 100% charge/discharge the capacity dropped to 97% after 300 cycles. 

At 80% it took over 1500 cycles. This is 5 times more cycles, but closer to 4 times improvement since only 80% capacity was available per cycle.  

At 20% it managed 3000 cycles. This sounds impressive, but since only 20% of the full capacity was extracted it is equivalent to far fewer than 3000 full charge/discharge cycles. Depending on how the device being powered uses the available voltage and capacity, it may even be worse than cycling to 80%. 







The main reason that a Lithium-ion battery 'wears out' is stress caused by expansion and contraction of the active material as lithium ions are squeezed in and out. Limiting the amount of charge/discharge reduces the expansion/contraction amplitude, which lessens damage to the plate material. 

However, even if a lithium-ion battery is held at constant state of charge the plates will still degrade due to oxidation. This occurs faster at higher temperature and voltage, so keeping your battery cool and at less than full charge will increase its shelf life. 

For a device such as a laptop which is usually used on mains power, it is better to run the battery down a bit and (if possible) remove it until it is needed. A device which is left unused for long periods should be stored with the battery partially discharged, and not recharged until just before the next use.