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 $58.29   13.729%   $66.2926   $66.29   $66.30   $66.30  
  Purchase #2 $245.62   9.814%   $269.7251   $269.73   $269.75   $269.70  
  Purchase #3 $187.34   13.531%   $212.689   $212.69   $212.70   $212.70  
  Purchase #4 $250.79   11.568%   $279.8014   $279.80   $279.80   $279.80  
  Purchase #5 $114.59   12.438%   $128.8427   $128.84   $128.85   $128.80  
  Purchase #6 $31.29   10.525%   $34.5833   $34.58   $34.60   $34.60  
  Purchase #7 $202.69   9.272%   $221.4834   $221.48   $221.50   $221.50  
  7 purchase totals = $1,090.61       $1,213.4175   $1,213.41   $1,213.50   $1,213.40