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 $37.77   5.845%   $39.9777   $39.98   $40.00   $40.00  
  Purchase #2 $20.73   11.680%   $23.1513   $23.15   $23.15   $23.20  
  Purchase #3 $74.74   11.751%   $83.5227   $83.52   $83.50   $83.50  
  Purchase #4 $202.31   6.235%   $214.924   $214.92   $214.90   $214.90  
  Purchase #5 $17.87   6.712%   $19.0694   $19.07   $19.05   $19.10  
  Purchase #6 $66.28   13.441%   $75.1887   $75.19   $75.20   $75.20  
  Purchase #7 $13.55   6.169%   $14.3859   $14.39   $14.40   $14.40  
  7 purchase totals = $433.25       $470.2197   $470.22   $470.20   $470.30