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 $208.39   11.581%   $232.5236   $232.52   $232.50   $232.50  
  Purchase #2 $250.42   9.555%   $274.3476   $274.35   $274.35   $274.30  
  Purchase #3 $202.79   12.201%   $227.5324   $227.53   $227.55   $227.50  
  Purchase #4 $113.68   4.865%   $119.2105   $119.21   $119.20   $119.20  
  Purchase #5 $8.49   5.830%   $8.985   $8.99   $9.00   $9.00  
  Purchase #6 $41.57   9.515%   $45.5254   $45.53   $45.55   $45.50  
  Purchase #7 $78.58   5.857%   $83.1824   $83.18   $83.20   $83.20  
  7 purchase totals = $903.92       $991.3069   $991.31   $991.35   $991.20