IMHO the best time to "check" them is by making sure you are VERY attentive when the booking is read back to you when it is made. If you feel it necessary repeat the date and time to the CM when they give you the confirmation number. That is the best time to catch any error because it is impossible for the CM to cancel the ressie at that point unless they use the confirmation number to reopen the reservation. If you call back to check you reservation, using the confirmation number, there is an option to cancel the reservation and it is relatively easy for the CM to cancel the ressie by mistake when trying to exit the system. If the reservation was correct originally when it was made and recapped to you, unless someone accesses that reservation, it will not change.
If you want to do a "quick check" for a number of reservations (time/date/restaurant) if you ask the CM to search for all of your dining reservations using your home phone number, they can see all the reservations you have and read that information to you, without having to access individual reservations. Again at this point it is impossible for them to cancel a ressie in error. It is only by looking at the individual reservation that it is possible for them to make a mistake.
Using your phone number will be MUCH quicker and has virtually no chance of a mistake being made. Using individual confirmation # s to check each one, one at a time , will take you 3 or 4 times as long to complete your check and it also means there is a VERY SLIGHT risk (maybe 1 in a 1000) that the CM will click on the wrong area and cancel the ressie without realizing it.
Just to underline the point, the best time to check the information is when you make the reservation by being attentive to the recap. I know it's easy to get distracted and bored at this point but the more times you fully access a reservation the higher risk you have of a mistake being made.