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 $18.50   10.657%   $20.4715   $20.47   $20.45   $20.50  
  Purchase #2 $170.94   10.102%   $188.2084   $188.21   $188.20   $188.20  
  Purchase #3 $201.25   4.427%   $210.1593   $210.16   $210.15   $210.20  
  Purchase #4 $97.28   6.642%   $103.7413   $103.74   $103.75   $103.70  
  Purchase #5 $121.40   9.566%   $133.0131   $133.01   $133.00   $133.00  
  Purchase #6 $69.64   6.489%   $74.1589   $74.16   $74.15   $74.20  
  Purchase #7 $134.42   8.136%   $145.3564   $145.36   $145.35   $145.40  
  7 purchase totals = $813.43       $875.1089   $875.11   $875.05   $875.20