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 $18.33   8.436%   $19.8763   $19.88   $19.90   $19.90  
  Purchase #2 $155.36   9.731%   $170.4781   $170.48   $170.50   $170.50  
  Purchase #3 $239.90   4.865%   $251.5711   $251.57   $251.55   $251.60  
  Purchase #4 $65.79   8.183%   $71.1736   $71.17   $71.15   $71.20  
  Purchase #5 $186.77   5.169%   $196.4241   $196.42   $196.40   $196.40  
  Purchase #6 $195.26   3.635%   $202.3577   $202.36   $202.35   $202.40  
  Purchase #7 $201.53   12.510%   $226.7414   $226.74   $226.75   $226.70  
  7 purchase totals = $1,062.94       $1,138.6223   $1,138.62   $1,138.60   $1,138.70