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 $91.79   4.606%   $96.0178   $96.02   $96.00   $96.00  
  Purchase #2 $96.31   9.332%   $105.2976   $105.30   $105.30   $105.30  
  Purchase #3 $126.36   7.690%   $136.0771   $136.08   $136.10   $136.10  
  Purchase #4 $205.52   13.156%   $232.5582   $232.56   $232.55   $232.60  
  Purchase #5 $67.58   9.815%   $74.213   $74.21   $74.20   $74.20  
  Purchase #6 $242.52   11.463%   $270.3201   $270.32   $270.30   $270.30  
  Purchase #7 $61.65   5.656%   $65.1369   $65.14   $65.15   $65.10  
  7 purchase totals = $891.73       $979.6207   $979.63   $979.60   $979.60