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 $165.75   13.440%   $188.0268   $188.03   $188.05   $188.00  
  Purchase #2 $51.47   6.593%   $54.8634   $54.86   $54.85   $54.90  
  Purchase #3 $39.55   12.782%   $44.6053   $44.61   $44.60   $44.60  
  Purchase #4 $165.39   11.199%   $183.912   $183.91   $183.90   $183.90  
  Purchase #5 $177.90   11.819%   $198.926   $198.93   $198.95   $198.90  
  Purchase #6 $26.40   6.164%   $28.0273   $28.03   $28.05   $28.00  
  Purchase #7 $44.89   12.286%   $50.4052   $50.41   $50.40   $50.40  
  7 purchase totals = $671.35       $748.7660   $748.78   $748.80   $748.70