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 $242.58   11.779%   $271.1535   $271.15   $271.15   $271.20  
  Purchase #2 $116.98   8.502%   $126.9256   $126.93   $126.95   $126.90  
  Purchase #3 $59.26   6.323%   $63.007   $63.01   $63.00   $63.00  
  Purchase #4 $229.65   12.164%   $257.5846   $257.58   $257.60   $257.60  
  Purchase #5 $37.97   13.561%   $43.1191   $43.12   $43.10   $43.10  
  Purchase #6 $42.79   8.833%   $46.5696   $46.57   $46.55   $46.60  
  Purchase #7 $16.48   3.357%   $17.0332   $17.03   $17.05   $17.00  
  7 purchase totals = $745.71       $825.3926   $825.39   $825.40   $825.40