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 $121.84   13.756%   $138.6003   $138.60   $138.60   $138.60  
  Purchase #2 $243.96   13.393%   $276.6336   $276.63   $276.65   $276.60  
  Purchase #3 $239.79   10.131%   $264.0831   $264.08   $264.10   $264.10  
  Purchase #4 $16.36   4.428%   $17.0844   $17.08   $17.10   $17.10  
  Purchase #5 $100.80   6.257%   $107.1071   $107.11   $107.10   $107.10  
  Purchase #6 $213.66   10.359%   $235.793   $235.79   $235.80   $235.80  
  Purchase #7 $60.34   10.779%   $66.844   $66.84   $66.85   $66.80  
  7 purchase totals = $996.75       $1,106.1455   $1,106.13   $1,106.20   $1,106.10