My friend tells me my car tires look low all the time?

I get them checked monthly and have a mechanic put air in them. they never say that my tires look bad or low. I am in florida where it is always stinkin hot...could this be why they tend to look low. my friend said Firestone stinks too so that could be why (he is not a mechanic) Anyone else have this problem? i have a 04 saturn ion 2. do i just need new tires??? :)