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 $234.55   12.706%   $264.3519   $264.35   $264.35   $264.40  
  Purchase #2 $137.29   10.745%   $152.0418   $152.04   $152.05   $152.00  
  Purchase #3 $12.86   5.327%   $13.5451   $13.55   $13.55   $13.50  
  Purchase #4 $157.48   4.268%   $164.2012   $164.20   $164.20   $164.20  
  Purchase #5 $83.78   3.810%   $86.972   $86.97   $86.95   $87.00  
  Purchase #6 $131.58   3.657%   $136.3919   $136.39   $136.40   $136.40  
  Purchase #7 $248.51   5.107%   $261.2014   $261.20   $261.20   $261.20  
  7 purchase totals = $1,006.05       $1,078.7053   $1,078.70   $1,078.70   $1,078.70