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 $16.99   4.871%   $17.8176   $17.82   $17.80   $17.80  
  Purchase #2 $87.29   4.232%   $90.9841   $90.98   $91.00   $91.00  
  Purchase #3 $227.26   8.804%   $247.268   $247.27   $247.25   $247.30  
  Purchase #4 $80.86   5.681%   $85.4537   $85.45   $85.45   $85.50  
  Purchase #5 $37.64   12.113%   $42.1993   $42.20   $42.20   $42.20  
  Purchase #6 $20.74   12.393%   $23.3103   $23.31   $23.30   $23.30  
  Purchase #7 $61.27   4.372%   $63.9487   $63.95   $63.95   $63.90  
  7 purchase totals = $532.05       $570.9817   $570.98   $570.95   $571.00