My main question - is it normal for it to back up a little as described above during these prolonged rainy periods?

For a modern system, no. For one as old as yours, it may not be normal but it's also not unexpected.

Thanks, then my next question would be if it's ok to proceed as usual with our daily wastewater routines without worry about a backup? An impossible question to answer perhaps. Just 2 old folks living here, our laundry is on a gray water system.

If you have not had problems before and I am sure there were heavy rainfalls in the past I am sorry to say that old system is on it's last legs. I don't think it will get much worse over time. It looks like time to replace the weeping bed.

"The septic system is a terra cotta (sp?) type, we've never have a problem with it..... "

I'm not an expert, but it sounds like you're describing a problem with it. Aren't you?

I'd think any situation where saturated ground leads to potential surface contamination from house sewage is 1) an issue under your local rules and 2) something you really should fix. Unless you really can't afford to.

I misstated should have said won't get better over time. Yes as snidely states raw sewage is on the ground surface, very bad health issue. I could not afford my repair but couldn't afford not to. I put in a holding tank that comes with an annual bill of about $800.00 for pumping, a new peat moss system was around $20000.00 yes twenty thousand.

Thanks for the feedback guys. To address two of your good points-
When I said "we never had a problem with it" I meant that there's never been a back-up into the house. The unvented toilet burping is evident only during extreme rain conditions such as the total of 6" fallen over the past few days on top of already rain saturated ground. Also there has Never been any evidence at all of septic leakage coming up to the ground surface, not ever in the 10 years we've lived here and I watch the ground closely, esp inspecting low puddling areas. The ground is very mushy but there's no sign of foul odors. Normal heavy rains = no problem. This type of flooding event which results in burping occurs about once per year. If the toilet was vented and didn't burp I'd probably never know or think that there was a "possible" problem. One thing I know for sure is that absolutely No sewage is coming to the surface, if it were I'd be asking other questions and getting estimates!

I once had surface dampness above one of two leach fields (it was past due time to alternate from one to the other, I'd forgotten to do it). There was no smell at all. I'd bet your mushy area has surface contamination from your septic system. I hope you don't have well water.

Oftentimes leach fields aren't allowed in yard areas with seasonal drainage problems. I've known people who had to put in uphill systems with pumps because of health department concern about lower-lying ground.