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 $25.45   12.292%   $28.5783   $28.58   $28.60   $28.60  
  Purchase #2 $143.42   12.305%   $161.0678   $161.07   $161.05   $161.10  
  Purchase #3 $129.51   3.736%   $134.3485   $134.35   $134.35   $134.30  
  Purchase #4 $25.54   3.850%   $26.5233   $26.52   $26.50   $26.50  
  Purchase #5 $10.57   12.199%   $11.8594   $11.86   $11.85   $11.90  
  Purchase #6 $25.96   10.527%   $28.6928   $28.69   $28.70   $28.70  
  Purchase #7 $226.54   8.273%   $245.2817   $245.28   $245.30   $245.30  
  7 purchase totals = $586.99       $636.3518   $636.35   $636.35   $636.40