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 $29.44   7.750%   $31.7216   $31.72   $31.70   $31.70  
  Purchase #2 $170.45   13.189%   $192.9307   $192.93   $192.95   $192.90  
  Purchase #3 $158.60   13.490%   $179.9951   $180.00   $180.00   $180.00  
  Purchase #4 $57.40   12.211%   $64.4091   $64.41   $64.40   $64.40  
  Purchase #5 $6.57   7.553%   $7.0662   $7.07   $7.05   $7.10  
  Purchase #6 $7.65   8.666%   $8.3129   $8.31   $8.30   $8.30  
  Purchase #7 $138.59   6.253%   $147.256   $147.26   $147.25   $147.30  
  7 purchase totals = $568.70       $631.6916   $631.70   $631.65   $631.70