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 $37.75   5.543%   $39.8425   $39.84   $39.85   $39.80  
  Purchase #2 $136.83   8.246%   $148.113   $148.11   $148.10   $148.10  
  Purchase #3 $134.72   7.682%   $145.0692   $145.07   $145.05   $145.10  
  Purchase #4 $7.28   12.753%   $8.2084   $8.21   $8.20   $8.20  
  Purchase #5 $134.82   7.778%   $145.3063   $145.31   $145.30   $145.30  
  Purchase #6 $55.37   9.527%   $60.6451   $60.65   $60.65   $60.60  
  Purchase #7 $76.74   6.536%   $81.7557   $81.76   $81.75   $81.80  
  7 purchase totals = $583.51       $628.9402   $628.95   $628.90   $628.90