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 $142.90   6.330%   $151.9456   $151.95   $151.95   $151.90  
  Purchase #2 $243.77   6.188%   $258.8545   $258.85   $258.85   $258.90  
  Purchase #3 $54.92   6.823%   $58.6672   $58.67   $58.65   $58.70  
  Purchase #4 $215.27   12.267%   $241.6772   $241.68   $241.70   $241.70  
  Purchase #5 $62.83   12.298%   $70.5568   $70.56   $70.55   $70.60  
  Purchase #6 $18.78   6.627%   $20.0246   $20.02   $20.00   $20.00  
  Purchase #7 $167.93   6.664%   $179.1209   $179.12   $179.10   $179.10  
  7 purchase totals = $906.40       $980.8468   $980.85   $980.80   $980.90