A rear-end height difference should not be a reason to cancel a front-end alignment, unless it's an extreme case. Sounds like BS to me from the tire/alignment place (maybe they don't know how to do it on an older car). Ask your trusty independent mechanic where he would take the car for an alignment. These cars are sensitive to proper alignment, and doing it is key to the fine handling we've all come to love and expect in these cars.
I think you have the SLS system in the SEC (you definitively have it in the SEL)... and if so, some height adjustment can be made by adjusting the rod that controls the valve at the rear. An easy DIY with a couple of 10mm (9mm??) wrenches. Do a search on Self Leveling Adjustment, and you're likely to find out whether it's lengthen the rod or shorten it (I can't remember). My front end does look higher than the rear from the clearance of the wheel wells, but the car rides level by reference to the sills under the doors. The fact that your (car's) rear end sags a bit is an indication of (A) an out-of-adjustment SLS, (B) worn springs (maybe), (C) worn rear subframe bushings (likely), (D) worn differential mount (also likely), or (E) faulty nitrogen cells (accumulators). In the latter case, your ride would be hard/bouncy, and if it isn't, it would probably eliminate the cells as a cause of the sag. Check the level of the hydraulic oil in the reservoir in the engine bay too.
I would definitely go with the subframe bushings and diff. mount first, as these are relatively inexpensive (like $50, total), and DIY-able in an hour or two. If you do decide to redo the springs, I would definitely go with H&Rs, and get Bilstein HD shocks for the front (figure $450 in parts). This upgrade represents a vast improvement in handling, with a little sacrifice in terms of ride harshness, from what I've read. If the rear hydraulics are OK (and you'd know it if they were leaking), I'd keep the stock system with the H&R upgrade. New accumulators are about $100 a side, while the hydraulic rams are very spendy ($800 each???), but rarely fail. You can ask Jonathan (member jhodg5ck) about his recommendations too.