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 $133.83   3.600%   $138.6479   $138.65   $138.65   $138.60  
  Purchase #2 $81.43   7.142%   $87.2457   $87.25   $87.25   $87.20  
  Purchase #3 $30.74   9.519%   $33.6661   $33.67   $33.65   $33.70  
  Purchase #4 $15.48   11.201%   $17.2139   $17.21   $17.20   $17.20  
  Purchase #5 $31.93   7.852%   $34.4371   $34.44   $34.45   $34.40  
  Purchase #6 $146.61   7.574%   $157.7142   $157.71   $157.70   $157.70  
  Purchase #7 $6.56   5.194%   $6.9007   $6.90   $6.90   $6.90  
  7 purchase totals = $446.58       $475.8256   $475.83   $475.80   $475.70