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 $28.82   4.691%   $30.1719   $30.17   $30.15   $30.20  
  Purchase #2 $115.37   5.385%   $121.5827   $121.58   $121.60   $121.60  
  Purchase #3 $124.74   11.150%   $138.6485   $138.65   $138.65   $138.60  
  Purchase #4 $90.39   5.832%   $95.6615   $95.66   $95.65   $95.70  
  Purchase #5 $141.56   13.297%   $160.3832   $160.38   $160.40   $160.40  
  Purchase #6 $130.37   10.843%   $144.506   $144.51   $144.50   $144.50  
  Purchase #7 $35.51   4.429%   $37.0827   $37.08   $37.10   $37.10  
  7 purchase totals = $666.76       $728.0365   $728.03   $728.05   $728.10