As mentioned, yes, you can upgrade a 3750 with copy tftp. If the 3750 you're upgrading is stand-alone, then you have no unusual concerns. But if, the 3750 is in a stack, then a "copy tftp" is only going to copy the bin to the master in the stack. If you then reboot that master, you'll find that he can't rejoin the stack because of the IOS version mismatch.
So for a 2-high stack, you could use the following process to upgrade the 3750's using copy tftp (lots of room for variation):
(1) Make sure you have enough room on flash to accept the new image, and make room if needed.
(2) Upgrade your master switch. Verify the image via checksum. Set your "boot system flash" parameter if necessary...usually not necessary.
(3) Make sure that you will still have connectivity to the stack if you power the newly upgraded switch down, and then power it off.
(4) The other switch will transition to stack master.
(5) You will lose your SSH/telnet connection to the stack because the master role moved to another stack member.
(6) Log back in, and upgrade the new master switch as above (verify your checksum, set boot system flash, etc.).
(7) Power off the second switch.
(8) Power up the whole stack. When the stack is completed reloading, you should have your shiny new IOS running on both stack members.
