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 $238.90   13.615%   $271.4262   $271.43   $271.45   $271.40  
  Purchase #2 $63.52   3.533%   $65.7642   $65.76   $65.75   $65.80  
  Purchase #3 $147.51   10.731%   $163.3393   $163.34   $163.35   $163.30  
  Purchase #4 $87.63   13.279%   $99.2664   $99.27   $99.25   $99.30  
  Purchase #5 $202.56   8.758%   $220.3002   $220.30   $220.30   $220.30  
  Purchase #6 $61.50   5.145%   $64.6642   $64.66   $64.65   $64.70  
  Purchase #7 $126.82   5.581%   $133.8978   $133.90   $133.90   $133.90  
  7 purchase totals = $928.44       $1,018.6583   $1,018.66   $1,018.65   $1,018.70