Are you talking about US equities, as a US citizen or resident?
If so, it probably won't make a difference, THIS YEAR, unless you have wash sales, which can jump into the next year even if you have no position at all on Dec 31. For FUTURE YEARS in which you have losses, it may make a difference, as there are "NOL carrybacks", so you aren't limited to writing off just the $3000 capital gains limit, etc.
There are many issues with switching to MTM, both positive and negative; the only way you'll get it figured out is by doing a lot of reading yourself, or by hiring an accountant who has expertise in this area (such as Robert Green, who posts here occasionally).
In any event, it's been my observation that EliteTrader is a horrible place for tax advice, so I'd rely on legitimate, outside sources for this type of info, rather than any posts by non-accountants (including this one).