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 $12.81   6.854%   $13.688   $13.69   $13.70   $13.70  
  Purchase #2 $47.69   13.582%   $54.1673   $54.17   $54.15   $54.20  
  Purchase #3 $173.92   8.525%   $188.7467   $188.75   $188.75   $188.70  
  Purchase #4 $80.83   8.400%   $87.6197   $87.62   $87.60   $87.60  
  Purchase #5 $78.64   6.603%   $83.8326   $83.83   $83.85   $83.80  
  Purchase #6 $128.67   9.818%   $141.3028   $141.30   $141.30   $141.30  
  Purchase #7 $140.57   13.499%   $159.5455   $159.55   $159.55   $159.50  
  7 purchase totals = $663.13       $728.9026   $728.91   $728.90   $728.80