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 $17.55   3.775%   $18.2125   $18.21   $18.20   $18.20  
  Purchase #2 $178.78   13.836%   $203.516   $203.52   $203.50   $203.50  
  Purchase #3 $196.84   6.193%   $209.0303   $209.03   $209.05   $209.00  
  Purchase #4 $45.56   12.634%   $51.3161   $51.32   $51.30   $51.30  
  Purchase #5 $245.85   7.468%   $264.2101   $264.21   $264.20   $264.20  
  Purchase #6 $123.37   13.546%   $140.0817   $140.08   $140.10   $140.10  
  Purchase #7 $36.60   11.527%   $40.8189   $40.82   $40.80   $40.80  
  7 purchase totals = $844.55       $927.1856   $927.19   $927.15   $927.10