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 $25.85   13.586%   $29.362   $29.36   $29.35   $29.40  
  Purchase #2 $16.51   11.859%   $18.4679   $18.47   $18.45   $18.50  
  Purchase #3 $145.34   9.212%   $158.7287   $158.73   $158.75   $158.70  
  Purchase #4 $39.96   12.627%   $45.0057   $45.01   $45.00   $45.00  
  Purchase #5 $106.64   7.358%   $114.4866   $114.49   $114.50   $114.50  
  Purchase #6 $109.45   9.696%   $120.0623   $120.06   $120.05   $120.10  
  Purchase #7 $198.48   9.112%   $216.5655   $216.57   $216.55   $216.60  
  7 purchase totals = $642.23       $702.6787   $702.69   $702.65   $702.80