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 $222.68   11.443%   $248.1613   $248.16   $248.15   $248.20  
  Purchase #2 $68.86   12.662%   $77.5791   $77.58   $77.60   $77.60  
  Purchase #3 $110.50   6.552%   $117.74   $117.74   $117.75   $117.70  
  Purchase #4 $78.39   5.461%   $82.6709   $82.67   $82.65   $82.70  
  Purchase #5 $216.55   13.292%   $245.3338   $245.33   $245.35   $245.30  
  Purchase #6 $121.96   12.350%   $137.0221   $137.02   $137.00   $137.00  
  Purchase #7 $21.40   7.239%   $22.9491   $22.95   $22.95   $22.90  
  7 purchase totals = $840.34       $931.4563   $931.45   $931.45   $931.40