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 $188.67   13.167%   $213.5122   $213.51   $213.50   $213.50  
  Purchase #2 $148.64   12.784%   $167.6421   $167.64   $167.65   $167.60  
  Purchase #3 $244.62   12.830%   $276.0047   $276.00   $276.00   $276.00  
  Purchase #4 $245.93   12.222%   $275.9876   $275.99   $276.00   $276.00  
  Purchase #5 $57.50   13.147%   $65.0595   $65.06   $65.05   $65.10  
  Purchase #6 $174.95   7.150%   $187.4589   $187.46   $187.45   $187.50  
  Purchase #7 $154.93   4.591%   $162.0428   $162.04   $162.05   $162.00  
  7 purchase totals = $1,215.24       $1,347.7078   $1,347.70   $1,347.70   $1,347.70