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 $138.28   9.531%   $151.4595   $151.46   $151.45   $151.50  
  Purchase #2 $216.26   7.485%   $232.4471   $232.45   $232.45   $232.40  
  Purchase #3 $159.47   7.209%   $170.9662   $170.97   $170.95   $171.00  
  Purchase #4 $59.27   12.377%   $66.6058   $66.61   $66.60   $66.60  
  Purchase #5 $7.93   6.301%   $8.4297   $8.43   $8.45   $8.40  
  Purchase #6 $96.90   3.175%   $99.9766   $99.98   $100.00   $100.00  
  Purchase #7 $234.40   12.832%   $264.4782   $264.48   $264.50   $264.50  
  7 purchase totals = $912.51       $994.3631   $994.38   $994.40   $994.40