Like the others said, a point in IF is less likely to do anything for you than the increased projectile coverage from an extra point in shield. 55 health (1 if) --> 57 (2 if) health doesn't create that many more situations where you'll be black barred instead of dead.
If you really plan on going 36+ getting full ranks in riding's probably a better choice, but getting IF/Shield pre-36 and buying training lessons at 36 is less annoying if you're impatient about the in-between levels.
Anyway, if you go with 18 STR:
Level 33 (37 332 277 xp)
Strength: 18
Agility: 24
Hit points: 53
Power Strike: 6
Shield(prioritize early shield usability): 4 --> 5 at 34
Shield(prioritize early cavalry usability): 4 --> 5 at 36
Athletics: 8
Riding(prioritize early shield usability): 0 --> 2 at 34 --> 5 at 35 --> 8 at 36
Riding(prioritize early cavalry usability): 0 --> 3 at 34 --> 6 at 35 --> 8 at 36
Weapon Master: 8
One Handed: 177 (use your own split)
Polearm: 96
Level 33 (37 332 277 xp)
Strength: 18
Agility: 24
Hit points: 53 -(if ironflesh picked)-> 57 at 35 --> 63 at 36
Riding or Ironflesh: 0 --> 2 at 35 --> 5 at 36
Power Strike: 6
Shield: 4 --> 7 at 34 --> 8 at 35
Athletics: 8
Weapon Master: 8
One Handed: 177 (use your own split)
Polearm: 96
Level 33 (37 332 277 xp)
Strength: 18
Agility: 24 --> 27 at 36
Hit points: 53
Power Strike: 6
Shield: 4 --> 5 at 34
Athletics: 8 --> 9 at 36
Weapon Master: 8 --> 9 at 36
One Handed: 177 --> 190 at 36 (use your own split)
Polearm: 96 --> 120 at 36