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 $206.96   11.874%   $231.5344   $231.53   $231.55   $231.50  
  Purchase #2 $248.72   5.689%   $262.8697   $262.87   $262.85   $262.90  
  Purchase #3 $150.43   3.657%   $155.9312   $155.93   $155.95   $155.90  
  Purchase #4 $71.54   5.638%   $75.5734   $75.57   $75.55   $75.60  
  Purchase #5 $244.49   9.151%   $266.8633   $266.86   $266.85   $266.90  
  Purchase #6 $122.83   11.212%   $136.6017   $136.60   $136.60   $136.60  
  Purchase #7 $173.96   5.388%   $183.333   $183.33   $183.35   $183.30  
  7 purchase totals = $1,218.93       $1,312.7067   $1,312.69   $1,312.70   $1,312.70