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 $28.98   6.586%   $30.8886   $30.89   $30.90   $30.90  
  Purchase #2 $31.28   12.664%   $35.2413   $35.24   $35.25   $35.20  
  Purchase #3 $46.73   12.345%   $52.4988   $52.50   $52.50   $52.50  
  Purchase #4 $135.76   13.276%   $153.7835   $153.78   $153.80   $153.80  
  Purchase #5 $33.87   4.137%   $35.2712   $35.27   $35.25   $35.30  
  Purchase #6 $168.84   4.336%   $176.1609   $176.16   $176.15   $176.20  
  Purchase #7 $121.41   8.341%   $131.5368   $131.54   $131.55   $131.50  
  7 purchase totals = $566.87       $615.3811   $615.38   $615.40   $615.40