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 $27.87   12.865%   $31.4555   $31.46   $31.45   $31.50  
  Purchase #2 $178.31   5.503%   $188.1224   $188.12   $188.10   $188.10  
  Purchase #3 $172.89   9.212%   $188.8166   $188.82   $188.80   $188.80  
  Purchase #4 $187.91   12.409%   $211.2278   $211.23   $211.25   $211.20  
  Purchase #5 $162.78   9.444%   $178.1529   $178.15   $178.15   $178.20  
  Purchase #6 $101.34   6.256%   $107.6798   $107.68   $107.70   $107.70  
  Purchase #7 $199.38   4.169%   $207.6922   $207.69   $207.70   $207.70  
  7 purchase totals = $1,030.48       $1,113.1472   $1,113.15   $1,113.15   $1,113.20