A recent article in the global news just came out about how the Americans may know that you have purchased legal medical pot and could deny you entry on those grounds.
Since we do not know for sure what the Americans can see, its best that when you are traveling to not say anything to the border agent unless they ask you. In other words, do not offer them any information regarding you being arrested or convicted unless asked.
If you have never been denied entry to the US then you likely do NOT need a US Entry Waiver. You really on need a US Waiver if you have been denied entry to the US. Once a pardon is granted, the Americans will not see your name on the CPIC or RCMP database so you should be safe to travel.
If you have been denied entry to the US then you will need to obtain a US Entry Waiver, I-192 and I-194 to be able to re-gain entry to the US. This process is expensive, complicated and takes time.
If you have been fingerprinted by the police and the charges were later dismissed, withdrawn, peace bond, stayed or discharged, its also important that you get a file destruction to remove your prints and photos, otherwise the Americans can see that you have been in trouble with the police and they will question you and possibly deny you entry to the US.
Here is the recent article in the Global News about how the Americans intend to continue to deny pot users and other criminals from entering the US: