Hello Kid_Icarus,
Welcome to the community!
While it is true that you will not get charged for incoming SMS, you should note that receiving (or sending) the data portion of a MMS outside of Canada would be considered roaming data and would be charged accordingly. You should also note that MMS data does not get transmitted via Wifi so it would still be considered roaming data even if you were connected to Wifi. Disabling data and/or roaming data should prevent those charges.
In addition, you should also note that rejecting or declining a call while outside of Canada is not the same as not answering the call. If you do not answer a call (ie let the call ring through), the call will get forwarded to your voicemail via the Fido networks. On the other hand, if you reject or decline a call when travelling, the call gets forwarded to your voicemail via a foreign network. The former would not incur roaming fees while the latter will likely incur fees.
As you note, an option to not receive incoming calls is to put your phone in Airplane mode and then enable Wifi when desired. Alternatively, some people also remove their SIMs prior to leaving Canada to also avoid roaming charges. Note, however, in either of those cases, you will not be able to receive SMS either.
Another option -- albeit slightly riskier -- is to set your phone to ring silently. Your phone will still be able to receive SMS and you won't be bothered by the ringing of calls. However, there's always the possibility that someone accidentally accepts or declines a call, thereby incurring roaming charges.
Hope this helps 
Cheers
