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 $50.78   11.165%   $56.4496   $56.45   $56.45   $56.40  
  Purchase #2 $214.69   8.776%   $233.5312   $233.53   $233.55   $233.50  
  Purchase #3 $181.89   12.599%   $204.8063   $204.81   $204.80   $204.80  
  Purchase #4 $204.95   4.777%   $214.7405   $214.74   $214.75   $214.70  
  Purchase #5 $55.82   6.337%   $59.3573   $59.36   $59.35   $59.40  
  Purchase #6 $165.60   3.669%   $171.6759   $171.68   $171.70   $171.70  
  Purchase #7 $51.40   12.378%   $57.7623   $57.76   $57.75   $57.80  
  7 purchase totals = $925.13       $998.3231   $998.33   $998.35   $998.30