Bytes you should never insert to the rom:

Miscellaneous: (You should never overwrite REP/SEP nor 1100 0000 (0x30) from its bits - other bits are fine usually)
00	=	BRK #$xx	2 bytes		Stack/Interrupt
02	=	COP #$xx	2 bytes		Stack/Interrupt
DB	=	STP		1 byte		Implied
CB	=	WAI		1 byte		Implied
C2	=	REP #$xx	2 bytes		Immediate
E2	=	SEP #$xx	2 byte		Immediate

Stack: (You should never overwrite any of these)
08	=	PHP		1 byte		Stack (Push)
0B	=	PHD		1 byte		Stack (Push)
8B	=	PHB		1 byte		Stack (Push)
4B	=	PHK		1 byte		Stack (Push)
48	=	PHA		1 byte		Stack (Push)
5A	=	PHY		1 byte		Stack (Push)
DA	=	PHX		1 byte		Stack (Push)
28	=	PLP		1 byte		Stack (Pull)
2B	=	PLD		1 byte		Stack (Pull)
AB	=	PLB		1 byte		Stack (Pull)
68	=	PLA		1 byte		Stack (Pull)
7A	=	PLY		1 byte		Stack (Pull)
FA	=	PLX		1 byte		Stack (Pull)
D4	=	PEI ($xx)	2 bytes		Stack (DP Indirect)
F4	=	PEA $xxxx	3 bytes		Stack (Absolute)
62	=	PER $xxxx	3 bytes		Stack (PC Relative Long)

Jump/Branch/Return: (You should never overwrite any of these)
80	=	BRA $xx		2 bytes		Program Counter Relative
82	=	BRL $xxxx	3 bytes		Program Counter Relative Long
30	=	BMI $xx		2 bytes		Program Counter Relative
50	=	BVC $xx		2 bytes		Program Counter Relative
70	=	BVS $xx		2 bytes		Program Counter Relative
90	=	BCC $xx		2 bytes		Program Counter Relative
B0	=	BCS $xx		2 bytes		Program Counter Relative
D0	=	BNE $xx		2 bytes		Program Counter Relative
F0	=	BEQ $xx		2 bytes		Program Counter Relative
10	=	BPL $xx		2 bytes		Program Counter Relative
4C	=	JMP $xxxx	3 bytes		Absolute
6C	=	JMP ($xxxx)	3 bytes		Absolute Indirect
DC	=	JMP [$xxxx]	3 bytes		Absolute Indirect Long
7C	=	JMP ($xxxx,x)	3 bytes		Absolute Indexed Indirect
5C	=	JML $xxxxxx	4 bytes		Absolute Long
20	=	JSR $xxxx	3 bytes		Absolute
FC	=	JSR ($xxxx,x)	3 bytes		Absolute Indexed Indirect
22	=	JSL $xxxxxx	4 bytes		Absolute Long
40	=	RTI		1 byte		Stack (RTI)
60	=	RTS		1 byte		Stack (RTS)
6B	=	RTL		1 byte		Stack (RTL)