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 $84.52   6.601%   $90.0992   $90.10   $90.10   $90.10  
  Purchase #2 $128.54   5.156%   $135.1675   $135.17   $135.15   $135.20  
  Purchase #3 $162.33   5.247%   $170.8475   $170.85   $170.85   $170.80  
  Purchase #4 $39.64   10.452%   $43.7832   $43.78   $43.80   $43.80  
  Purchase #5 $39.62   9.506%   $43.3863   $43.39   $43.40   $43.40  
  Purchase #6 $19.35   10.263%   $21.3359   $21.34   $21.35   $21.30  
  Purchase #7 $160.87   13.633%   $182.8014   $182.80   $182.80   $182.80  
  7 purchase totals = $634.87       $687.4210   $687.43   $687.45   $687.40