As many have mentioned bottle neck rifle cases don't need to be flared when using jacketed bullet a good chamfer will do just fine. Jacketed bullet are smaller in dia. and have a harder copper jacket so seating those with the standard 30-30 die set is never an issue.
Cast bullet on the other hand are softer and larger in dia. general .001 to .002" flaring the case mouth on bottle neck rifle case when using a cast bullet is the common practice with both plain base and gas check bullets especially with a plain base bullets. Shaving lead off the base or sides of a cast bullet just opens you up to several opportunities for leading to occur by imparting defects in the bullet and reducing it's dia.
For low velocity plinking loads where high pressure and high velocity isn't an issue you may not notice anything but if your shooting both high pressure and high velocity 30-30 plain base cast bullet load those defects can open the door for gas cutting and loss of the lube seal which can cause leading.
I flair all my bottle neck rifle cases that I load cast bullets in using the Lee Universal flaring die,flaring the case allows the bullet to seat easier without shaving any lead off the bullet. I also use the appropriate size expander ball to resize the case neck so as not to downsize the softer cast bullet. The .311 dia. espander ball from my RDBS 54r resizing die works perfectly fine for neck expansion in all my 30 cal. rifle cases when using both plain base and gas check bullets.
I use two primary loads in my 30-30 using the Lee gas check 170 gr. RNFP I cast myself,the first is my general purpose plinking and small game load,bullets are cast from straight WW alloy BHN 12 bullets drop form the mold at just a hair over .310 dia. they get two light coats of Lee Alox/JPW lube no gas check is applied and I push them at 1200+ fps. with 8.0 grs. of Alliant Red Dot
Lyman Cast Bullet manual data.
For my full house hunting loads I cast the same bullet from a 30:1 Lead/Tin alloy BHN 9,gas check are applied with a little imperial sizing wax on my fingers and a .311 Lee sizer then ranched dipped in Alox/JPW lube to fill the lube grooves. After drying overnight the bullets are push back through the sizer gas check first to clean off the excess lube which also leaves the nose nice and clean and doesn't gum up the bullet seater die. I push these bullets at just a little over 1900 fps. using 28 grs. of H335 both loads are as accurate as you can hold them and cost about 8 to 12 cents each to produce.
A few of my ranch dipped and sized bullets.
L Lee 170 gr. GC RNFP .310
M Lee 160 gr. GC TL RN .314
R Lee 158 gr. GC SWC .359