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 $21.48   12.302%   $24.1225   $24.12   $24.10   $24.10  
  Purchase #2 $199.73   7.519%   $214.7477   $214.75   $214.75   $214.70  
  Purchase #3 $188.71   7.290%   $202.467   $202.47   $202.45   $202.50  
  Purchase #4 $167.87   5.366%   $176.8779   $176.88   $176.90   $176.90  
  Purchase #5 $86.60   7.809%   $93.3626   $93.36   $93.35   $93.40  
  Purchase #6 $183.91   5.823%   $194.6191   $194.62   $194.60   $194.60  
  Purchase #7 $100.98   5.363%   $106.3956   $106.40   $106.40   $106.40  
  7 purchase totals = $949.28       $1,012.5924   $1,012.60   $1,012.55   $1,012.60