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 $153.77   12.173%   $172.4884   $172.49   $172.50   $172.50  
  Purchase #2 $96.70   6.863%   $103.3365   $103.34   $103.35   $103.30  
  Purchase #3 $62.53   13.539%   $70.9959   $71.00   $71.00   $71.00  
  Purchase #4 $19.71   12.402%   $22.1544   $22.15   $22.15   $22.20  
  Purchase #5 $29.84   6.145%   $31.6737   $31.67   $31.65   $31.70  
  Purchase #6 $83.29   12.788%   $93.9411   $93.94   $93.95   $93.90  
  Purchase #7 $134.46   9.373%   $147.0629   $147.06   $147.05   $147.10  
  7 purchase totals = $580.30       $641.6529   $641.65   $641.65   $641.70