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 $12.60   3.866%   $13.0871   $13.09   $13.10   $13.10  
  Purchase #2 $139.76   7.695%   $150.5145   $150.51   $150.50   $150.50  
  Purchase #3 $204.40   7.274%   $219.2681   $219.27   $219.25   $219.30  
  Purchase #4 $91.27   13.473%   $103.5668   $103.57   $103.55   $103.60  
  Purchase #5 $121.39   11.226%   $135.0172   $135.02   $135.00   $135.00  
  Purchase #6 $203.76   6.672%   $217.3549   $217.35   $217.35   $217.40  
  Purchase #7 $11.39   13.394%   $12.9156   $12.92   $12.90   $12.90  
  7 purchase totals = $784.57       $851.7242   $851.73   $851.65   $851.80