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 $20.60   3.421%   $21.3047   $21.30   $21.30   $21.30  
  Purchase #2 $123.51   5.812%   $130.6884   $130.69   $130.70   $130.70  
  Purchase #3 $45.81   8.755%   $49.8207   $49.82   $49.80   $49.80  
  Purchase #4 $160.45   11.771%   $179.3366   $179.34   $179.35   $179.30  
  Purchase #5 $7.78   3.187%   $8.0279   $8.03   $8.05   $8.00  
  Purchase #6 $15.36   9.334%   $16.7937   $16.79   $16.80   $16.80  
  Purchase #7 $118.69   11.512%   $132.3536   $132.35   $132.35   $132.40  
  7 purchase totals = $492.20       $538.3256   $538.32   $538.35   $538.30