Is and iz
One of the pronunciation rules says:
After one of the sibilant sounds Is/, Izl, /JV, /3/, /tj/ and IA3I, the plural ending -es is pronounced /iz/.
buses /'bASiz/ crashes /'krae/iz/ watches /'wnt jiz/ quizzes /'kwiziz/ garages /'gaera:3iz/ bridges /'brid^iz/

--------

Does it make any difference if I pronounce the end for all words ending with "s" as "s" and not "z" because there's no way I will remember what noun to pronounce which? Will you still understand?
Will there be any case where not pronouncing "iz" makes it sound like a different word? But they don't sound that different, right?

Question

Is and iz
One of the pronunciation rules says: 
After one of the sibilant sounds Is/, Izl, /JV, /3/, /tj/ and IA3I, the plural ending -es is pronounced /iz/.
buses /'bASiz/ crashes /'krae/iz/ watches /'wnt jiz/ quizzes /'kwiziz/ garages /'gaera:3iz/ bridges /'brid^iz/

--------

Does it make any difference if I pronounce the end for all words ending with "s" as "s" and not "z" because there's no way I will remember what noun to pronounce which? Will you still understand? 
 Will there be any case where not pronouncing "iz" makes it sound like a different word? But they don't sound that different, right?

Answer

The rules for how to pronounce the plural suffix -es are exactly the same as the rules for how to pronounce the verb suffix -es.

The noun "watches" and the verb "watches" are homonyms: they have the same pronunciation, "watch-iz". Saying "watch-iss" usually sounds like the speaker has a foreign accent.

Pronouncing the last sound like "s" will make you sound like you have a non-native accent.