Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $19.28   8.757%   $20.9683   $20.97   $20.95   $21.00  
  Purchase #2 $79.75   12.281%   $89.5441   $89.54   $89.55   $89.50  
  Purchase #3 $171.39   10.523%   $189.4254   $189.43   $189.45   $189.40  
  Purchase #4 $249.48   4.743%   $261.3128   $261.31   $261.30   $261.30  
  Purchase #5 $24.32   3.128%   $25.0807   $25.08   $25.10   $25.10  
  Purchase #6 $81.54   12.114%   $91.4178   $91.42   $91.40   $91.40  
  Purchase #7 $21.79   11.421%   $24.2786   $24.28   $24.30   $24.30  
  7 purchase totals = $647.55       $702.0277   $702.03   $702.05   $702.00