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 $246.96   13.795%   $281.0281   $281.03   $281.05   $281.00  
  Purchase #2 $182.30   6.378%   $193.9271   $193.93   $193.95   $193.90  
  Purchase #3 $248.87   5.179%   $261.759   $261.76   $261.75   $261.80  
  Purchase #4 $26.59   13.611%   $30.2092   $30.21   $30.20   $30.20  
  Purchase #5 $80.47   13.796%   $91.5716   $91.57   $91.55   $91.60  
  Purchase #6 $49.47   8.111%   $53.4825   $53.48   $53.50   $53.50  
  Purchase #7 $160.42   4.627%   $167.8426   $167.84   $167.85   $167.80  
  7 purchase totals = $995.08       $1,079.8201   $1,079.82   $1,079.85   $1,079.80