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 $28.87   3.850%   $29.9815   $29.98   $30.00   $30.00  
  Purchase #2 $36.64   4.696%   $38.3606   $38.36   $38.35   $38.40  
  Purchase #3 $168.52   11.822%   $188.4424   $188.44   $188.45   $188.40  
  Purchase #4 $98.43   3.148%   $101.5286   $101.53   $101.55   $101.50  
  Purchase #5 $191.56   5.589%   $202.2663   $202.27   $202.25   $202.30  
  Purchase #6 $112.61   8.381%   $122.0478   $122.05   $122.05   $122.00  
  Purchase #7 $37.55   4.655%   $39.298   $39.30   $39.30   $39.30  
  7 purchase totals = $674.18       $721.9252   $721.93   $721.95   $721.90