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 $120.66   13.687%   $137.1747   $137.17   $137.15   $137.20  
  Purchase #2 $170.39   3.464%   $176.2923   $176.29   $176.30   $176.30  
  Purchase #3 $124.86   3.269%   $128.9417   $128.94   $128.95   $128.90  
  Purchase #4 $25.37   5.760%   $26.8313   $26.83   $26.85   $26.80  
  Purchase #5 $56.46   7.266%   $60.5624   $60.56   $60.55   $60.60  
  Purchase #6 $55.28   5.294%   $58.2065   $58.21   $58.20   $58.20  
  Purchase #7 $115.37   11.199%   $128.2903   $128.29   $128.30   $128.30  
  7 purchase totals = $668.39       $716.2992   $716.29   $716.30   $716.30