As I just asked a question on Excel, I saw that the tag spreadsheet had 3 questions (which are also about excel) and that excel has 50 questions.
It feels redundant.
Obviously there are more spreadsheets than just excel, so I would leave the tag there. There will be questions about open office calc at some point.
Tagging systems are there to help find questions. They categorize questions. And they are community driven, so let the community decide.
If these tags were duplicates (excel, ms-excel, microsoft-excel), I would fold them together again to help finding questions.
As tags are suggested when we start typing in the tag list, the spreadsheet tag being available will push people in the direction of using it when writing about any spreadsheet. Maybe they already found it when searching for their question and use it therefore.
If it is omitted now, old questions will not be tagged accordingly in the future when others reinvent the tag.
So leave it, as the community has decided to use it.
This would be like saying all photocopies are Xeroxes, that all tissues are Kleenexes.
Just because there's a Microsoft product for spreadsheets called Excel doesn't mean that all spreadsheets are Excel.
OpenOffice.org has their own spreadsheet software, Calc. Should questions on Calc be retagged as Excel?