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 $10.91   9.825%   $11.9819   $11.98   $12.00   $12.00  
  Purchase #2 $31.86   13.804%   $36.258   $36.26   $36.25   $36.30  
  Purchase #3 $238.33   7.161%   $255.3968   $255.40   $255.40   $255.40  
  Purchase #4 $128.64   7.561%   $138.3665   $138.37   $138.35   $138.40  
  Purchase #5 $170.50   6.415%   $181.4376   $181.44   $181.45   $181.40  
  Purchase #6 $85.46   11.738%   $95.4913   $95.49   $95.50   $95.50  
  Purchase #7 $156.82   9.659%   $171.9672   $171.97   $171.95   $172.00  
  7 purchase totals = $822.52       $890.8993   $890.91   $890.90   $891.00