The current author of Optifine says that you should open RAM with less memory if you want to increase FPS. I'm no programmer but I'm pretty sure he knows what he's talking about.
And the reason it could do that, is if you set it to use more RAM than you have free - in which case it'll start using the pagefile, and slow down everything instead.
Starting Minecraft with more allowed RAM should be no issue at all if you have the free RAM - I'm having no issues, regardless of letting it have more, or default.
Some people get crashes because they don't have enough RAM allocated. Depends on computer and mods that you're using. This fix is required to play for longer than 10 minutes for some people (including myself).
i use this line in my bat file using a 64bit OS and 64bit java with 4 gigs of ram, i allow minecraft 3gigs
javaw -Xmx3072m -Xms2048m -jar "Minecraft.exe"
if you use a 32 bit OS you'll be limited to lower amount even if you have more physical ram, i think it maxes out at around 1400mb that you can use, so it'd be something like
javaw -Xmx1400m -Xms1024m -jar "Minecraft.exe"
i have the .bat file and .exe both on my desktop so thats y i don't need "C:/"
if you want to see all the stuff loaded in the console for minecraft or help with displaying errors that crash minecraft all you need to do is remove the "w" from "javaw" and add a pause, so it looks like this