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 $139.88   13.565%   $158.8547   $158.85   $158.85   $158.90  
  Purchase #2 $6.51   7.521%   $6.9996   $7.00   $7.00   $7.00  
  Purchase #3 $9.46   3.476%   $9.7888   $9.79   $9.80   $9.80  
  Purchase #4 $55.47   9.404%   $60.6864   $60.69   $60.70   $60.70  
  Purchase #5 $7.84   6.867%   $8.3784   $8.38   $8.40   $8.40  
  Purchase #6 $237.95   6.526%   $253.4786   $253.48   $253.50   $253.50  
  Purchase #7 $124.78   8.604%   $135.5161   $135.52   $135.50   $135.50  
  7 purchase totals = $581.89       $633.7026   $633.71   $633.75   $633.80