19. Always 19. Tome of strength will not boost up to str percentiles, it just skips straight to 19, and is thus very powerful to use on any character with str 18
19. 18/xx STR exists only for warrior classes, non-warriors go straight from plain 18 to 19 (which is indeed a big jump). It's not usually hugely significant for mages since they're not usually combat-orientated, but it can be a pretty major boost for priests and rogues who are more likely to be making melee attacks.
For melee-oriented but non-fighter kits like Swashbucklers and Blades, it's always worth trying to go for 18 Str, to get to 19 Str after the tome, which can really help with any THAC0 problems (and deal much greater damage, of course).
For a Blade you can get away with only 13 Int, if you glug a potion or two of Genius before a batch scroll-scribing session, and this still gives you 9 spells in your book per level, which is plenty enough - this means that you can use those spare points to put into Strength instead e.g. my current blade started out with 18/18/15/13/7/15, which is perfectly usable, but required a roll of only 86 (I tend to limit myself to a max 20 re-rolls on character creation to avoid overpowered characters, so need to make the most of what rolls I get). The 15 con and 15 Cha can both be brought up to 16 very early in the game and you can reach 10 Wis to avoid the -10 lore penalty, if you use all three of the Wis tomes on CHARNAME., though obviously not until very late in the game (still useful for BG2EE, though)_.
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For a Blade you can get away with only 13 Int, if you glug a potion or two of Genius before a batch scroll-scribing session, and this still gives you 9 spells in your book per level, which is plenty enough - this means that you can use those spare points to put into Strength instead e.g. my current blade started out with 18/18/15/13/7/15, which is perfectly usable, but required a roll of only 86 (I tend to limit myself to a max 20 re-rolls on character creation to avoid overpowered characters, so need to make the most of what rolls I get). The 15 con and 15 Cha can both be brought up to 16 very early in the game and you can reach 10 Wis to avoid the -10 lore penalty, if you use all three of the Wis tomes on CHARNAME., though obviously not until very late in the game (still useful for BG2EE, though)_.