Technically any standard hashing algorithms is not reversible! so once you get the hash of a message, there should be no way to get the original message out of its hash string. The only way people try to crack it is to use brute force attack. Brute forcing is the stupidest thing you can do, trying all possible keys! That explains why one of the characteristic of a secure cryptographic algorithm is to have a large key space. But if you utilize the process in some cases it can be practical, that's what exactly rainbow tables do.
A rainbow table is a precomputed table for all possible combinations up to a certain length. That means you create all possible combination of characters(upper case and lower case), numbers and special characters up to certain length. As far as I know the most complete rainbow table can break hash of strings up to 10 characters which include numbers, uppercase and special characters, so if you have string longer than that there shouldn't be any security concern about breaking the hash itself. as you can see here the size of the table that can break vista passwords up to 8 characters is over 100GB and that number increases exponentially which makes it kind of impossible to go further that 10 or 12 characters.
As long as your string is not easy to guess, long enough and it contains upper case letters, numbers and special characters, there no need to worry :)
