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 $172.30   8.590%   $187.1006   $187.10   $187.10   $187.10  
  Purchase #2 $40.91   8.537%   $44.4025   $44.40   $44.40   $44.40  
  Purchase #3 $84.63   6.512%   $90.1411   $90.14   $90.15   $90.10  
  Purchase #4 $213.79   12.189%   $239.8489   $239.85   $239.85   $239.80  
  Purchase #5 $120.73   6.833%   $128.9795   $128.98   $129.00   $129.00  
  Purchase #6 $162.70   3.400%   $168.2318   $168.23   $168.25   $168.20  
  Purchase #7 $27.86   11.548%   $31.0773   $31.08   $31.10   $31.10  
  7 purchase totals = $822.92       $889.7817   $889.78   $889.85   $889.70