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 $219.32   12.559%   $246.8644   $246.86   $246.85   $246.90  
  Purchase #2 $113.75   5.585%   $120.1029   $120.10   $120.10   $120.10  
  Purchase #3 $181.93   13.608%   $206.687   $206.69   $206.70   $206.70  
  Purchase #4 $21.32   8.820%   $23.2004   $23.20   $23.20   $23.20  
  Purchase #5 $45.87   10.388%   $50.635   $50.64   $50.65   $50.60  
  Purchase #6 $230.75   5.829%   $244.2004   $244.20   $244.20   $244.20  
  Purchase #7 $71.47   11.204%   $79.4775   $79.48   $79.50   $79.50  
  7 purchase totals = $884.41       $971.1676   $971.17   $971.20   $971.20