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 $160.65   3.330%   $165.9996   $166.00   $166.00   $166.00  
  Purchase #2 $206.46   9.764%   $226.6188   $226.62   $226.60   $226.60  
  Purchase #3 $240.30   13.109%   $271.8009   $271.80   $271.80   $271.80  
  Purchase #4 $211.37   8.229%   $228.7636   $228.76   $228.75   $228.80  
  Purchase #5 $231.34   3.766%   $240.0523   $240.05   $240.05   $240.10  
  Purchase #6 $18.28   3.766%   $18.9684   $18.97   $18.95   $19.00  
  Purchase #7 $175.82   3.872%   $182.6278   $182.63   $182.65   $182.60  
  7 purchase totals = $1,244.22       $1,334.8314   $1,334.83   $1,334.80   $1,334.90