Do I need hiberfil.sys if I only use "Sleep" on a Windows 7 instance that's on an SSD?

You should not delete this file. Sleep and Hibernate work together in Windows 7 and Windows 8.

Windows 7 has a mode named hybrid sleep, used on desktop computers. To quote Microsoft:

Hybrid sleep is designed primarily for desktop computers. Hybrid sleep is a combination of sleep and hibernate—it puts any open documents and programs in memory and on your hard disk, and then puts your computer into a low-power state so that you can quickly resume your work. That way, if a power failure occurs, Windows can restore your work from your hard disk. When hybrid sleep is turned on, putting your computer into sleep automatically puts your computer into hybrid sleep. Hybrid sleep is typically turned on by default on desktop computers.

Also, on laptops, if your laptop battery charge gets critically low while the computer is asleep, Windows automatically puts the laptop into hibernation mode.

Therefore, do not delete this file, unless you turn off both Sleep and Hibernation in your active power plan.

For more information on Sleep and Hibernation, I recommend reading this FAQ: Sleep and hibernation: frequently asked questions.

hiberfil.sys is used by Hibernate, not by Sleep. The only surefire method of disabling Hibernate, and thus removing hiberfil.sys, on Windows Vista and Windows 7, is through the command prompt and the following steps:

1. Open a command prompt with administrative privileges.
2. Enter “powercfg.exe -h off”.
3. Exit the command prompt.

The accepted answer by alexs was -almost- right for me, with only the small variation that I had to use slash instead of minus:

powercfg /h off

OS: Windows 7 Professional

Please see the accepted answer here for more details: Using Powercfg to remove Hibernation file does not work , even though I am the Administrator

You can change the size of hiberfile.sys, on Win7-Win10 by opening an elevated command prompt

• press the windows key
• type "cmd"
• right-click on "cmd" in the list that pops up
• choose "run as administrator"

then type the the command,

Powercfg –h –size percentsize 

replace percentsize with your value (for example, 60%), but it cannot be smaller than 50. See here for some pics.