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 $137.41   13.744%   $156.2956   $156.30   $156.30   $156.30  
  Purchase #2 $244.98   11.855%   $274.0224   $274.02   $274.00   $274.00  
  Purchase #3 $203.90   11.222%   $226.7817   $226.78   $226.80   $226.80  
  Purchase #4 $35.58   5.133%   $37.4063   $37.41   $37.40   $37.40  
  Purchase #5 $114.66   9.591%   $125.657   $125.66   $125.65   $125.70  
  Purchase #6 $83.86   11.697%   $93.6691   $93.67   $93.65   $93.70  
  Purchase #7 $39.42   9.486%   $43.1594   $43.16   $43.15   $43.20  
  7 purchase totals = $859.81       $956.9915   $957.00   $956.95   $957.10