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 $172.45   7.754%   $185.8218   $185.82   $185.80   $185.80  
  Purchase #2 $26.73   13.216%   $30.2626   $30.26   $30.25   $30.30  
  Purchase #3 $35.70   9.838%   $39.2122   $39.21   $39.20   $39.20  
  Purchase #4 $34.88   6.185%   $37.0373   $37.04   $37.05   $37.00  
  Purchase #5 $28.35   8.212%   $30.6781   $30.68   $30.70   $30.70  
  Purchase #6 $124.28   13.500%   $141.0578   $141.06   $141.05   $141.10  
  Purchase #7 $160.64   9.492%   $175.8879   $175.89   $175.90   $175.90  
  7 purchase totals = $583.03       $639.9577   $639.96   $639.95   $640.00