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 $30.49   4.189%   $31.7672   $31.77   $31.75   $31.80  
  Purchase #2 $6.74   4.438%   $7.0391   $7.04   $7.05   $7.00  
  Purchase #3 $166.54   11.603%   $185.8636   $185.86   $185.85   $185.90  
  Purchase #4 $18.49   8.106%   $19.9888   $19.99   $20.00   $20.00  
  Purchase #5 $184.98   6.250%   $196.5413   $196.54   $196.55   $196.50  
  Purchase #6 $142.95   12.871%   $161.3491   $161.35   $161.35   $161.30  
  Purchase #7 $86.59   8.459%   $93.9146   $93.91   $93.90   $93.90  
  7 purchase totals = $636.78       $696.4637   $696.46   $696.45   $696.40