Your host do say they have 99.95% uptime .. guaranteed. Well I believe 99.99% (my host) uptime a month is around 48 mins max downtime (i think) - I did work it out one rainy-internet-down-day. 
So if it's been down more than an hour or something then you defenately need to tell them... unless the problem is caused by "her" ("Mother Nature") (eg; Flood, Lightning Strikes, Earthquake, Exploding Ducks) then there's not much that can be done.
So if it's been down more than an hour or something then you defenately need to tell them... unless the problem is caused by "her" ("Mother Nature") (eg; Flood, Lightning Strikes, Earthquake, Exploding Ducks) then there's not much that can be done.