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 $214.62   12.674%   $241.8209   $241.82   $241.80   $241.80  
  Purchase #2 $100.33   4.319%   $104.6633   $104.66   $104.65   $104.70  
  Purchase #3 $131.92   5.138%   $138.698   $138.70   $138.70   $138.70  
  Purchase #4 $19.27   3.218%   $19.8901   $19.89   $19.90   $19.90  
  Purchase #5 $138.26   12.693%   $155.8093   $155.81   $155.80   $155.80  
  Purchase #6 $24.37   6.168%   $25.8731   $25.87   $25.85   $25.90  
  Purchase #7 $53.96   5.855%   $57.1194   $57.12   $57.10   $57.10  
  7 purchase totals = $682.73       $743.8741   $743.87   $743.80   $743.90