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 $77.62   8.500%   $84.2177   $84.22   $84.20   $84.20  
  Purchase #2 $223.84   13.175%   $253.3309   $253.33   $253.35   $253.30  
  Purchase #3 $70.79   10.692%   $78.3589   $78.36   $78.35   $78.40  
  Purchase #4 $165.61   4.423%   $172.9349   $172.93   $172.95   $172.90  
  Purchase #5 $131.36   5.675%   $138.8147   $138.81   $138.80   $138.80  
  Purchase #6 $207.48   7.269%   $222.5617   $222.56   $222.55   $222.60  
  Purchase #7 $73.70   7.615%   $79.3123   $79.31   $79.30   $79.30  
  7 purchase totals = $950.40       $1,029.5311   $1,029.52   $1,029.50   $1,029.50