Gravel wshers are only good if what is in the gravel is inert at the time that you begin using it.
Meaning if you are pulling out large peices of junk, dead plants, leaves, fish, etc. you would be ok.
However, you should understand that if this is replacing your regular water changes, this is a bad idea. Yes, it will put out the junk, but it puts back in the tank the ammonia and nitrites that may be present in your tank. Also the very fine stuff makes its way back into the tank and back into gravel or suspended until the filter gets to it. Assuming it is not too small to get caught by the filter cartridge.
I use the one that actually removes the water from the tank along with the junk.
So I guess the answer is part yes and part no.
Yes if your water quality if good, no ammonia and no nitrites and you just need to get out large debri.
No, if you are looking to get rid of doing your regular water changes or if your water quality is not up to snuff.
Remember it is always what you can not see that kills your fish.