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 $82.36   8.763%   $89.5772   $89.58   $89.60   $89.60  
  Purchase #2 $223.84   12.439%   $251.6835   $251.68   $251.70   $251.70  
  Purchase #3 $62.66   10.506%   $69.2431   $69.24   $69.25   $69.20  
  Purchase #4 $195.31   3.672%   $202.4818   $202.48   $202.50   $202.50  
  Purchase #5 $14.36   10.586%   $15.8801   $15.88   $15.90   $15.90  
  Purchase #6 $61.69   11.122%   $68.5512   $68.55   $68.55   $68.60  
  Purchase #7 $177.51   12.690%   $200.036   $200.04   $200.05   $200.00  
  7 purchase totals = $817.73       $897.4529   $897.45   $897.55   $897.50