First, we'll need to specify the partitioning scheme used. The classic one for PCs is the MBR, which shares the name with the very first block of the disk (logical block address #0) in this partitioning scheme: the Master Boot Record. So, the answer to the question title for MBR partitioning is yes.
The Master Boot Record contains a maximum of 446 bytes of boot code, the partition table for primary partitions (only 16 bytes per partition) and a boot signature (2 bytes). Of each 16-byte partition entry, only 4 bytes are actually useable for specifying the first block of a partition on modern disks, and only 4 bytes for specifying the number of blocks in a partition. As a result, the starting point of a MBR partition must be within (2^32 - 1) blocks from the beginning of the disk, and the size of a MBR partition must be (2^32 - 1) blocks or less. In practice, this limits the usefulness of MBR partitioning to disks of 2 TB or less.
Traditionally, the MBR boot code would just identify the active primary partition, load the first block of that partition (sometimes called a Partition Boot Record, or PBR) and execute it. The PBR would then contain an operating-system-specific boot program. But modern bootloaders, like GRUB for Linux, or some full-disk encryption solutions, may wish to do more complex things and so won't fit in a single block.
In an old disk, back when the Cylinder/Head/Sector geometry values actually had some actual meaning, the entire first track of the first head of the disk was reserved for the MBR block alone: the convention was to start each partition at the beginning of a track, so as track #0 had its first block occupied by the MBR, the entire track was skipped, and the first partition would start at track #1 instead. As a result, the sectors on track #0 after the MBR block were available for use by more advanced bootloaders. On a MBR-partitioned disk, the GRUB bootloader will usually embed at least some parts of itself in there.
On modern disks, where LBA addressing is the norm, a new convention was established: the first partition would normally start at logical block address #2048, or exactly 1 MiB from the beginning of the disk. This would ensure that even if the disk would internally handle the blocks in groups of some power-of-two, the start of the first partition would still be aligned with the beginning of such a block group. On RAID arrays and other larger storage systems, such data alignment can be an important part for getting full performance out of the storage. As a side effect, this new convention was likely to cause even more "wasted" space between the MBR block and the beginning of the first partition.
In 2011, when the Intel Sandy Bridge microarchitecture was released, the new UEFI firmware was introduced to the masses, to eventually replace the traditional BIOS. Along with it, came a new partitioning scheme: GUID Partition Table or GPT for short. Since the MBR had a pretty fundamental maximum size limit of 2 terabytes (assuming the standard block size of 512 bytes), a new scheme was needed.
The GPT partitioning scheme actually contains a "protective MBR": the very first block (LBA #0) of a GPT-partitioned disk contains a valid MBR block that essentially says "this disk has been fully allocated to a partition type you don't know about, don't touch" to systems that only understand MBR-style partitioning. Starting from LBA #1 is then the actual partition table data of the GPT. It allows partition sizes to grow into the zettabytes range - that should be plenty for the foreseeable future.
The GPT partitioning scheme does away with the restrictions of MBR:
- With GPT, there is no longer a limit of only 4 primary partitions per disk.
- The MBR-style division of primary/extended/logical partitions no longer exists: all partitions are equal in that sense.
The UEFI firmware also specifies a new bootloader scheme. The bootloader no longer has a fixed location on the disk. Instead, the firmware will have a built-in capacity to read files from a FAT32-type filesystem, and a bootloader will be just a regular file on a FAT32 partition marked with a specific partition type GUID. Such a partition is called ESP, or EFI system partition. (UEFI firmware standard was developed from EFI firmware, which existed mainly on Intel Itanium systems, and the name and structure of the bootloader partition was inherited from there.)
So, for other partitioning schemes, including GPT, you cannot assume the boot sector is the first block on the drive - in fact, you cannot assume that the concept of a boot sector even exists!