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 $80.91   12.596%   $91.1014   $91.10   $91.10   $91.10  
  Purchase #2 $48.34   4.501%   $50.5158   $50.52   $50.50   $50.50  
  Purchase #3 $45.57   9.388%   $49.8481   $49.85   $49.85   $49.80  
  Purchase #4 $74.62   8.837%   $81.2142   $81.21   $81.20   $81.20  
  Purchase #5 $41.59   8.129%   $44.9709   $44.97   $44.95   $45.00  
  Purchase #6 $21.25   3.789%   $22.0552   $22.06   $22.05   $22.10  
  Purchase #7 $78.95   4.710%   $82.6685   $82.67   $82.65   $82.70  
  7 purchase totals = $391.23       $422.3741   $422.38   $422.30   $422.40