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 $20.99   5.328%   $22.1083   $22.11   $22.10   $22.10  
  Purchase #2 $66.40   13.155%   $75.1349   $75.13   $75.15   $75.10  
  Purchase #3 $180.77   4.258%   $188.4672   $188.47   $188.45   $188.50  
  Purchase #4 $211.60   3.582%   $219.1795   $219.18   $219.20   $219.20  
  Purchase #5 $122.93   9.391%   $134.4744   $134.47   $134.45   $134.50  
  Purchase #6 $250.45   3.834%   $260.0523   $260.05   $260.05   $260.10  
  Purchase #7 $90.73   10.724%   $100.4599   $100.46   $100.45   $100.50  
  7 purchase totals = $943.87       $999.8765   $999.87   $999.85   $1,000.00