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 $110.66   9.839%   $121.5478   $121.55   $121.55   $121.50  
  Purchase #2 $86.31   4.416%   $90.1214   $90.12   $90.10   $90.10  
  Purchase #3 $190.73   6.440%   $203.013   $203.01   $203.00   $203.00  
  Purchase #4 $20.75   8.815%   $22.5791   $22.58   $22.60   $22.60  
  Purchase #5 $157.41   6.352%   $167.4087   $167.41   $167.40   $167.40  
  Purchase #6 $60.47   7.292%   $64.8795   $64.88   $64.90   $64.90  
  Purchase #7 $38.95   3.699%   $40.3908   $40.39   $40.40   $40.40  
  7 purchase totals = $665.28       $709.9403   $709.94   $709.95   $709.90