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 $8.46   11.388%   $9.4234   $9.42   $9.40   $9.40  
  Purchase #2 $72.83   11.185%   $80.976   $80.98   $81.00   $81.00  
  Purchase #3 $204.28   13.561%   $231.9824   $231.98   $232.00   $232.00  
  Purchase #4 $184.50   4.445%   $192.701   $192.70   $192.70   $192.70  
  Purchase #5 $224.43   12.634%   $252.7845   $252.78   $252.80   $252.80  
  Purchase #6 $11.29   3.130%   $11.6434   $11.64   $11.65   $11.60  
  Purchase #7 $87.84   3.764%   $91.1463   $91.15   $91.15   $91.10  
  7 purchase totals = $793.63       $870.6570   $870.65   $870.70   $870.60