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 $17.71   3.579%   $18.3438   $18.34   $18.35   $18.30  
  Purchase #2 $42.48   11.142%   $47.2131   $47.21   $47.20   $47.20  
  Purchase #3 $130.65   11.861%   $146.1464   $146.15   $146.15   $146.10  
  Purchase #4 $13.88   8.556%   $15.0676   $15.07   $15.05   $15.10  
  Purchase #5 $147.49   12.558%   $166.0118   $166.01   $166.00   $166.00  
  Purchase #6 $66.61   8.573%   $72.3205   $72.32   $72.30   $72.30  
  Purchase #7 $194.55   12.866%   $219.5808   $219.58   $219.60   $219.60  
  7 purchase totals = $613.37       $684.6840   $684.68   $684.65   $684.60