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 $5.33   3.426%   $5.5126   $5.51   $5.50   $5.50  
  Purchase #2 $36.79   8.617%   $39.9602   $39.96   $39.95   $40.00  
  Purchase #3 $153.33   10.854%   $169.9724   $169.97   $169.95   $170.00  
  Purchase #4 $90.36   10.661%   $99.9933   $99.99   $100.00   $100.00  
  Purchase #5 $126.43   9.496%   $138.4358   $138.44   $138.45   $138.40  
  Purchase #6 $132.89   4.269%   $138.5631   $138.56   $138.55   $138.60  
  Purchase #7 $6.65   6.296%   $7.0687   $7.07   $7.05   $7.10  
  7 purchase totals = $551.78       $599.5061   $599.50   $599.45   $599.60