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 $222.45   4.218%   $231.8329   $231.83   $231.85   $231.80  
  Purchase #2 $121.59   10.630%   $134.515   $134.52   $134.50   $134.50  
  Purchase #3 $220.96   12.165%   $247.8398   $247.84   $247.85   $247.80  
  Purchase #4 $241.51   7.378%   $259.3286   $259.33   $259.35   $259.30  
  Purchase #5 $160.28   12.140%   $179.738   $179.74   $179.75   $179.70  
  Purchase #6 $145.30   8.598%   $157.7929   $157.79   $157.80   $157.80  
  Purchase #7 $201.90   3.640%   $209.2492   $209.25   $209.25   $209.20  
  7 purchase totals = $1,313.99       $1,420.2964   $1,420.30   $1,420.35   $1,420.10