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 $7.53   7.444%   $8.0905   $8.09   $8.10   $8.10  
  Purchase #2 $120.26   12.541%   $135.3418   $135.34   $135.35   $135.30  
  Purchase #3 $190.83   10.601%   $211.0599   $211.06   $211.05   $211.10  
  Purchase #4 $20.93   10.697%   $23.1689   $23.17   $23.15   $23.20  
  Purchase #5 $75.26   7.584%   $80.9677   $80.97   $80.95   $81.00  
  Purchase #6 $121.88   8.303%   $131.9997   $132.00   $132.00   $132.00  
  Purchase #7 $48.37   6.802%   $51.6601   $51.66   $51.65   $51.70  
  7 purchase totals = $585.06       $642.2886   $642.29   $642.25   $642.40