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 $34.51   11.751%   $38.5653   $38.57   $38.55   $38.60  
  Purchase #2 $60.83   7.185%   $65.2006   $65.20   $65.20   $65.20  
  Purchase #3 $33.82   12.532%   $38.0583   $38.06   $38.05   $38.10  
  Purchase #4 $151.53   7.181%   $162.4114   $162.41   $162.40   $162.40  
  Purchase #5 $108.60   12.220%   $121.8709   $121.87   $121.85   $121.90  
  Purchase #6 $62.79   12.672%   $70.7467   $70.75   $70.75   $70.70  
  Purchase #7 $167.48   6.364%   $178.1384   $178.14   $178.15   $178.10  
  7 purchase totals = $619.56       $674.9916   $675.00   $674.95   $675.00