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 $242.94   6.618%   $259.0178   $259.02   $259.00   $259.00  
  Purchase #2 $111.86   9.106%   $122.046   $122.05   $122.05   $122.00  
  Purchase #3 $218.56   12.835%   $246.6122   $246.61   $246.60   $246.60  
  Purchase #4 $46.60   7.504%   $50.0969   $50.10   $50.10   $50.10  
  Purchase #5 $125.81   6.692%   $134.2292   $134.23   $134.25   $134.20  
  Purchase #6 $119.35   3.310%   $123.3005   $123.30   $123.30   $123.30  
  Purchase #7 $56.89   3.340%   $58.7901   $58.79   $58.80   $58.80  
  7 purchase totals = $922.01       $994.0927   $994.10   $994.10   $994.00