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 $6.70   6.588%   $7.1414   $7.14   $7.15   $7.10  
  Purchase #2 $28.44   4.236%   $29.6447   $29.64   $29.65   $29.60  
  Purchase #3 $152.64   6.280%   $162.2258   $162.23   $162.25   $162.20  
  Purchase #4 $17.72   12.138%   $19.8709   $19.87   $19.85   $19.90  
  Purchase #5 $117.91   3.687%   $122.2573   $122.26   $122.25   $122.30  
  Purchase #6 $31.30   9.159%   $34.1668   $34.17   $34.15   $34.20  
  Purchase #7 $246.43   8.395%   $267.1178   $267.12   $267.10   $267.10  
  7 purchase totals = $601.14       $642.4247   $642.43   $642.40   $642.40