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 $68.98   12.463%   $77.577   $77.58   $77.60   $77.60  
  Purchase #2 $102.98   9.379%   $112.6385   $112.64   $112.65   $112.60  
  Purchase #3 $137.67   9.791%   $151.1493   $151.15   $151.15   $151.10  
  Purchase #4 $241.35   11.460%   $269.0087   $269.01   $269.00   $269.00  
  Purchase #5 $79.30   8.762%   $86.2483   $86.25   $86.25   $86.20  
  Purchase #6 $171.27   5.522%   $180.7275   $180.73   $180.75   $180.70  
  Purchase #7 $105.66   6.613%   $112.6473   $112.65   $112.65   $112.60  
  7 purchase totals = $907.21       $989.9966   $990.01   $990.05   $989.80