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 $117.53   9.581%   $128.7905   $128.79   $128.80   $128.80  
  Purchase #2 $128.54   7.780%   $138.5404   $138.54   $138.55   $138.50  
  Purchase #3 $40.84   10.710%   $45.214   $45.21   $45.20   $45.20  
  Purchase #4 $131.28   3.381%   $135.7186   $135.72   $135.70   $135.70  
  Purchase #5 $35.39   3.355%   $36.5773   $36.58   $36.60   $36.60  
  Purchase #6 $119.36   3.841%   $123.9446   $123.94   $123.95   $123.90  
  Purchase #7 $72.71   9.380%   $79.5302   $79.53   $79.55   $79.50  
  7 purchase totals = $645.65       $688.3156   $688.31   $688.35   $688.20