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 $121.34   13.640%   $137.8908   $137.89   $137.90   $137.90  
  Purchase #2 $146.69   10.499%   $162.091   $162.09   $162.10   $162.10  
  Purchase #3 $108.45   7.299%   $116.3658   $116.37   $116.35   $116.40  
  Purchase #4 $144.65   12.364%   $162.5345   $162.53   $162.55   $162.50  
  Purchase #5 $147.25   4.384%   $153.7054   $153.71   $153.70   $153.70  
  Purchase #6 $39.77   10.799%   $44.0648   $44.06   $44.05   $44.10  
  Purchase #7 $194.70   4.177%   $202.8326   $202.83   $202.85   $202.80  
  7 purchase totals = $902.85       $979.4849   $979.48   $979.50   $979.50