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 $233.92   13.539%   $265.5904   $265.59   $265.60   $265.60  
  Purchase #2 $142.29   9.594%   $155.9413   $155.94   $155.95   $155.90  
  Purchase #3 $202.74   13.156%   $229.4125   $229.41   $229.40   $229.40  
  Purchase #4 $35.72   10.753%   $39.561   $39.56   $39.55   $39.60  
  Purchase #5 $39.29   12.524%   $44.2107   $44.21   $44.20   $44.20  
  Purchase #6 $32.62   4.559%   $34.1071   $34.11   $34.10   $34.10  
  Purchase #7 $248.55   4.173%   $258.922   $258.92   $258.90   $258.90  
  7 purchase totals = $935.13       $1,027.7450   $1,027.74   $1,027.70   $1,027.70