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 $250.46   13.118%   $283.3153   $283.32   $283.30   $283.30  
  Purchase #2 $10.38   10.375%   $11.4569   $11.46   $11.45   $11.50  
  Purchase #3 $85.58   8.104%   $92.5154   $92.52   $92.50   $92.50  
  Purchase #4 $240.89   11.322%   $268.1636   $268.16   $268.15   $268.20  
  Purchase #5 $30.69   3.115%   $31.646   $31.65   $31.65   $31.60  
  Purchase #6 $218.62   11.695%   $244.1876   $244.19   $244.20   $244.20  
  Purchase #7 $103.49   3.424%   $107.0335   $107.03   $107.05   $107.00  
  7 purchase totals = $940.11       $1,038.3183   $1,038.33   $1,038.30   $1,038.30