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 $232.81   8.157%   $251.8003   $251.80   $251.80   $251.80  
  Purchase #2 $57.99   5.463%   $61.158   $61.16   $61.15   $61.20  
  Purchase #3 $94.66   5.822%   $100.1711   $100.17   $100.15   $100.20  
  Purchase #4 $33.66   11.622%   $37.572   $37.57   $37.55   $37.60  
  Purchase #5 $7.34   11.522%   $8.1857   $8.19   $8.20   $8.20  
  Purchase #6 $106.71   4.356%   $111.3583   $111.36   $111.35   $111.40  
  Purchase #7 $25.54   3.144%   $26.343   $26.34   $26.35   $26.30  
  7 purchase totals = $558.71       $596.5884   $596.59   $596.55   $596.70