I have no preference for this but what I would recommend is that all tempo marks and any instruction that changes tempo are all confined to one specific staff, not necessarily dedicated. Markings that that change tempi will not work unless they have a tempo mark before and after the change marking.
This applies to markings such as accel, rit, ral etc.
To see this in action, play this as is. Then move the accel marking to the first staff and play again.
!NoteWorthyComposer(2.5)
|Locale|ACP:1252
|Editor|ActiveStaff:1|CaretIndex:1|CaretPos:2
|SongInfo|Title:""|Author:"<Name>"|Lyricist:""|Copyright1:"Copyright © 2012 <Name>"|Copyright2:"All Rights Reserved"
|PgSetup|StaffSize:16|Zoom:4|TitlePage:Y|JustifyVertically:Y|PrintSystemSepMark:N|ExtendLastSystem:N|DurationPadding:Y|PageNumbers:0|StaffLabels:None|BarNumbers:None|StartingBar:1
|Font|Style:StaffItalic|Typeface:"Times New Roman"|Size:10|Bold:Y|Italic:Y|CharSet:0
|Font|Style:StaffBold|Typeface:"Times New Roman"|Size:8|Bold:Y|Italic:N|CharSet:0
|Font|Style:StaffLyric|Typeface:"Times New Roman"|Size:7|Bold:N|Italic:N|CharSet:0
|Font|Style:PageTitleText|Typeface:"Times New Roman"|Size:24|Bold:Y|Italic:N|CharSet:0
|Font|Style:PageText|Typeface:"Times New Roman"|Size:12|Bold:N|Italic:N|CharSet:0
|Font|Style:PageSmallText|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User1|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User2|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User3|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User4|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User5|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User6|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|PgMargins|Left:1.27|Top:1.27|Right:1.27|Bottom:1.27|Mirror:N
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:12|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:1
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Tempo|Tempo:120|Pos:12
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Tempo|Tempo:240|Pos:12
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Note|Dur:4th|Pos:0
|Bar
|AddStaff|Name:"Staff-1"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:10|BoundaryBottom:10|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:2
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Clef|Type:Treble
|TimeSig|Signature:4/4
|TempoVariance|Style:Accelerando|Pos:7
|Rest|Dur:Whole
|Bar
|RestMultiBar|NumBars:8|PrintOnce:Y|WhenHidden:ShowBars,ShowRests
!NoteWorthyComposer-End