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 $250.75   13.560%   $284.7517   $284.75   $284.75   $284.80  
  Purchase #2 $80.89   8.820%   $88.0245   $88.02   $88.00   $88.00  
  Purchase #3 $109.98   4.467%   $114.8928   $114.89   $114.90   $114.90  
  Purchase #4 $241.80   6.795%   $258.2303   $258.23   $258.25   $258.20  
  Purchase #5 $221.63   11.225%   $246.508   $246.51   $246.50   $246.50  
  Purchase #6 $218.57   6.823%   $233.483   $233.48   $233.50   $233.50  
  Purchase #7 $203.79   7.715%   $219.5124   $219.51   $219.50   $219.50  
  7 purchase totals = $1,327.41       $1,445.4027   $1,445.39   $1,445.40   $1,445.40