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 $121.69   5.283%   $128.1189   $128.12   $128.10   $128.10  
  Purchase #2 $48.63   5.225%   $51.1709   $51.17   $51.15   $51.20  
  Purchase #3 $187.70   13.694%   $213.4036   $213.40   $213.40   $213.40  
  Purchase #4 $105.57   3.264%   $109.0158   $109.02   $109.00   $109.00  
  Purchase #5 $57.44   9.774%   $63.0542   $63.05   $63.05   $63.10  
  Purchase #6 $80.95   12.240%   $90.8583   $90.86   $90.85   $90.90  
  Purchase #7 $208.29   9.831%   $228.767   $228.77   $228.75   $228.80  
  7 purchase totals = $810.27       $884.3887   $884.39   $884.30   $884.50