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 $108.31   8.162%   $117.1503   $117.15   $117.15   $117.20  
  Purchase #2 $241.94   11.331%   $269.3542   $269.35   $269.35   $269.40  
  Purchase #3 $127.67   10.208%   $140.7026   $140.70   $140.70   $140.70  
  Purchase #4 $164.30   9.867%   $180.5115   $180.51   $180.50   $180.50  
  Purchase #5 $6.67   10.225%   $7.352   $7.35   $7.35   $7.40  
  Purchase #6 $24.36   10.186%   $26.8413   $26.84   $26.85   $26.80  
  Purchase #7 $98.36   12.852%   $111.0012   $111.00   $111.00   $111.00  
  7 purchase totals = $771.61       $852.9131   $852.90   $852.90   $853.00