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 $233.99   5.686%   $247.2947   $247.29   $247.30   $247.30  
  Purchase #2 $31.72   13.790%   $36.0942   $36.09   $36.10   $36.10  
  Purchase #3 $74.84   7.693%   $80.5974   $80.60   $80.60   $80.60  
  Purchase #4 $30.74   11.293%   $34.2115   $34.21   $34.20   $34.20  
  Purchase #5 $171.77   9.717%   $188.4609   $188.46   $188.45   $188.50  
  Purchase #6 $144.42   4.817%   $151.3767   $151.38   $151.40   $151.40  
  Purchase #7 $244.36   3.715%   $253.438   $253.44   $253.45   $253.40  
  7 purchase totals = $931.84       $991.4734   $991.47   $991.50   $991.50