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 $250.55   8.153%   $270.9773   $270.98   $271.00   $271.00  
  Purchase #2 $189.79   6.567%   $202.2535   $202.25   $202.25   $202.30  
  Purchase #3 $166.41   5.840%   $176.1283   $176.13   $176.15   $176.10  
  Purchase #4 $153.53   6.865%   $164.0698   $164.07   $164.05   $164.10  
  Purchase #5 $99.89   10.730%   $110.6082   $110.61   $110.60   $110.60  
  Purchase #6 $172.75   5.753%   $182.6883   $182.69   $182.70   $182.70  
  Purchase #7 $162.75   5.627%   $171.9079   $171.91   $171.90   $171.90  
  7 purchase totals = $1,195.67       $1,278.6333   $1,278.64   $1,278.65   $1,278.70