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 $15.37   10.597%   $16.9988   $17.00   $17.00   $17.00  
  Purchase #2 $118.40   8.194%   $128.1017   $128.10   $128.10   $128.10  
  Purchase #3 $99.94   5.247%   $105.1839   $105.18   $105.20   $105.20  
  Purchase #4 $45.57   11.539%   $50.8283   $50.83   $50.85   $50.80  
  Purchase #5 $122.61   8.433%   $132.9497   $132.95   $132.95   $132.90  
  Purchase #6 $153.75   4.846%   $161.2007   $161.20   $161.20   $161.20  
  Purchase #7 $39.84   12.620%   $44.8678   $44.87   $44.85   $44.90  
  7 purchase totals = $595.48       $640.1309   $640.13   $640.15   $640.10