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 $79.99   4.442%   $83.5432   $83.54   $83.55   $83.50  
  Purchase #2 $101.29   3.641%   $104.978   $104.98   $105.00   $105.00  
  Purchase #3 $26.47   6.395%   $28.1628   $28.16   $28.15   $28.20  
  Purchase #4 $32.29   7.717%   $34.7818   $34.78   $34.80   $34.80  
  Purchase #5 $38.84   10.854%   $43.0557   $43.06   $43.05   $43.10  
  Purchase #6 $236.78   7.722%   $255.0642   $255.06   $255.05   $255.10  
  Purchase #7 $6.55   3.418%   $6.7739   $6.77   $6.75   $6.80  
  7 purchase totals = $522.21       $556.3596   $556.35   $556.35   $556.50