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 $8.81   12.223%   $9.8868   $9.89   $9.90   $9.90  
  Purchase #2 $5.50   5.803%   $5.8192   $5.82   $5.80   $5.80  
  Purchase #3 $54.89   5.510%   $57.9144   $57.91   $57.90   $57.90  
  Purchase #4 $15.96   8.482%   $17.3137   $17.31   $17.30   $17.30  
  Purchase #5 $198.46   10.332%   $218.9649   $218.96   $218.95   $219.00  
  Purchase #6 $41.32   8.393%   $44.788   $44.79   $44.80   $44.80  
  Purchase #7 $228.83   6.212%   $243.0449   $243.04   $243.05   $243.00  
  7 purchase totals = $553.77       $597.7319   $597.72   $597.70   $597.70