Wikipedia has a tone contour chart for Cantonese with only 6 tones, but the table just below has 9. In fact, the table states 7=1, 8=3 and 9=6. From a historical point of view, there are 9 tone classes in Cantonese. From a practical point of view, does it make sense to distinguish the checked tones or is the only difference between them and their flat equivalents the fact they are carried by syllables with stop codas whereas the equivalents are carried by syllables with other types of codas (none, semivowel /j/ and /w/, nasal codas)?
From a practical learner's point of view, treating the checked "tones" as shorter, closed syllables that carry the same tone as as tones 1, 3, 6 (and 2 in changed tone) would be enough. In modern Cantonese of the Pearl River Delta, there appears to be little to no difference in pitch between the non-checked and checked.
In Taishanese, one of the checked tones matches to a different tone to standard Yuehai Cantonese. Also in some other non-Yue varieties, checked tones in tone sandhi behave very differently to non-checked tones, even if they have the same contour in isolation (e.g. Xiamen/Amoy Min Nan). So it is possible for checked syllables to have different patterns: it's just not a feature of standard Cantonese.
1Yep, that's pretty much right. Interestingly, checked tones with long vowels (e.g. 百 baak3) may be longer or as long as non-checked syllables, but that final consonant is usually clear enough. Sep 15, 2014 at 16:29