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 $243.37   13.655%   $276.6022   $276.60   $276.60   $276.60  
  Purchase #2 $235.61   5.413%   $248.3636   $248.36   $248.35   $248.40  
  Purchase #3 $120.96   3.127%   $124.7424   $124.74   $124.75   $124.70  
  Purchase #4 $210.73   12.246%   $236.536   $236.54   $236.55   $236.50  
  Purchase #5 $225.33   6.866%   $240.8012   $240.80   $240.80   $240.80  
  Purchase #6 $238.82   8.260%   $258.5465   $258.55   $258.55   $258.50  
  Purchase #7 $115.42   10.302%   $127.3106   $127.31   $127.30   $127.30  
  7 purchase totals = $1,390.24       $1,512.9025   $1,512.90   $1,512.90   $1,512.80