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 $37.35   7.361%   $40.0993   $40.10   $40.10   $40.10  
  Purchase #2 $79.35   7.391%   $85.2148   $85.21   $85.20   $85.20  
  Purchase #3 $28.90   8.656%   $31.4016   $31.40   $31.40   $31.40  
  Purchase #4 $36.81   8.708%   $40.0154   $40.02   $40.00   $40.00  
  Purchase #5 $118.85   12.478%   $133.6801   $133.68   $133.70   $133.70  
  Purchase #6 $13.79   6.672%   $14.7101   $14.71   $14.70   $14.70  
  Purchase #7 $215.64   10.712%   $238.7394   $238.74   $238.75   $238.70  
  7 purchase totals = $530.69       $583.8607   $583.86   $583.85   $583.80