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 $220.58   3.120%   $227.4621   $227.46   $227.45   $227.50  
  Purchase #2 $7.88   13.650%   $8.9556   $8.96   $8.95   $9.00  
  Purchase #3 $138.67   6.667%   $147.9151   $147.92   $147.90   $147.90  
  Purchase #4 $170.61   10.863%   $189.1434   $189.14   $189.15   $189.10  
  Purchase #5 $106.53   5.122%   $111.9865   $111.99   $112.00   $112.00  
  Purchase #6 $202.47   6.287%   $215.1993   $215.20   $215.20   $215.20  
  Purchase #7 $18.55   4.448%   $19.3751   $19.38   $19.40   $19.40  
  7 purchase totals = $865.29       $920.0371   $920.05   $920.05   $920.10