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 $221.75   13.435%   $251.5421   $251.54   $251.55   $251.50  
  Purchase #2 $22.78   13.154%   $25.7765   $25.78   $25.80   $25.80  
  Purchase #3 $193.49   12.321%   $217.3299   $217.33   $217.35   $217.30  
  Purchase #4 $165.39   8.402%   $179.2861   $179.29   $179.30   $179.30  
  Purchase #5 $140.70   13.134%   $159.1795   $159.18   $159.20   $159.20  
  Purchase #6 $190.99   8.157%   $206.5691   $206.57   $206.55   $206.60  
  Purchase #7 $51.74   3.578%   $53.5913   $53.59   $53.60   $53.60  
  7 purchase totals = $986.84       $1,093.2745   $1,093.28   $1,093.35   $1,093.30